For Denmark you can have a look at "Jyske Lov" or Codex Holmiensis, I can't find any english translations, but basically you were the investigator to a crime committed against you. In the case of a murder, you would typically have an idea who it was (very few people get murdered for no reason), and you could sue for the murder at the local "thing" (assembly).

I'm going off of this modern language version: http://www.middelalderinfo.dk/jyskelov-bog3.php

However, in some cases of murder, the king would have a right for a claim as well as the injured person:

Part 22 book 3.

"Dræber en Mand en anden i Leding eller i en Forsamling eller paa Tinge eller paa Tingvej eller i sit eget Hus eller i et Herred, hvor Kongen opholder sig, eller i Kirken eller paa Kirkegaarden eller i Købstaden eller med Kniv eller en Bonde, der holder paa sin Plov, da skal Drabsmanden foruden ret Mandebod altid bøde fyrretyve Mark til den dræbtes Arvinger og ligeledes til Kongen, hvis han beholder sin Fred og betaler Bøder."

"If a man kills another, in the militia, or in a gathering, or at the thing, or on a kings road, or in his own house, or in a shire where the king presides, or in the church, or in the cemetary, or in the merchant city, or with a knife, or a farmer who is holding his plough, then shall the murderer in excess of his fine for murder, always be fine forty marks to the victims inheritors and also to the king, if he maintains his peace and pay fines"

Basically, if a man was killed anywhere while doing anything, the king had a claim to the fine, and his local representative would be able to investigate and sue.

How investigations worked is probably best represented by the section on theft, particularly chapter 95 book 2 "How to search for your stolen property":

"Hvorledes man skal søge efter sine stjaalne Ejendele.

Mister en Mand sine Ejendele, da skal han følge efter til den By, hvor han mener, at hans Ejendele er kommet hen, og lyse for alle Granderne efter det, han har mistet. Ind°©rømmer nogen Mand, (at han har) de Ejendele, og siger, af hvem han har faaet dem, da er han fritaget for Tyvsnavn, hvis den, han fik dem af, tilstaar. Men hvis en vejfarende Mand, som han ikke kendte, lagde eller efterlod de Ejendele i hans Hus eller overgav ham dem til Opbevaring, da er han pligtig at bevise med tolv Mænds Ed, at han ikke er Tyv og ikke Tyvs Medvider og aldrig har lagt Skjul paa, (at han havde) de Ejendele..."

"How to search for your stolen property.

If a man loses his property, then he should follow it to the city, where he believes his property has been taken to, and ask all property owners about what he has lost. If any man admits that he has those properties, and says who he has received them from, then he is excempt from being called a thief, if the person he got them from, confesses. But if a travelling man, who he did not know, laid or left those properties in his house, or gave them to him for safekeeping, then he is obligated to prove with twelve men's oath, that he is not a thief and not a thief's accomplice, and never has hidden that he had those properties..."

However, if nobody has your stolen goods, and a further investigation is required, you can move to chapter 96 book 2:

"Hvis ingen i Byen indrømmer, (at han har) de Ejendele.

Nægter alle, der bor i Byen, (at de har) de Ejendele, som 30 man lyser efter, da kan han forlange at foretage Ransagning over hele Byen, og de kan ikke nægte ham det; men en enkelt eller to Mænds Gaarde maa en Mand ikke ransage, medmindre han stiller sine tre Mark i Sikkerhed for det Tilfælde, at han ikke finder Tyvekosterne derinde. Men naar man tillader, at der ransages over hele Byen, da skal Ombudsmanden overvære det, hvis han er til Stede. Er han ikke til Stede, da kan Bønderne selv ransage og begynde, hvor i Byen de vil, og hvor de end finder Kosterne, som den bestjaalne har efterlyst, da kan vedkommende, hvis de findes under Husbondens eller Hustruens Laas, ikke senere give en anden Skylden derfor, fordi de tidligere har nægtet, og da er han Tyv med Hensyn til de Koster og har dermed forbrudt sin Hals..."

"If no-one in the city admit that they have the property.

If all who live int he city deny that they have the property that 30 man are looking for, then he can demand to do an inspection of the whole town, and they can not deny it to him; but a single or two mans farms are not allowed to be inspected, unless he puts is three mark as warranty for the case that they does not find the goods there. But when you allow that the whole town is inspected, then the kings representative shall be there, if he is around. If he is not around then the farmers, then the farmers shall inspect themselves and start wherever they will, and where they find the goods under lock, the farmer or his wife can no longer give anyone else the blame for it, as they have earlier claimed not to have stolen it..."

Also relating to theft, but what to do if there is a suspicion of theft, and you sue a man based on that suspicion. part 106 book 2:

"Hvorledes man skal gaa frem i en Sag, der hviler paa Mistanke.

Ved en Mand, hvem Tyven er, og dog ikke kan faa sine Ejendele at se, da skal han rejse Sag mod den Mand, som han formoder er skyldig i den Sag, og lade ham stævne til Tinget, saaledes som det tidligere er sagt om Ranssag, og paa det andet Ting skal han aflægge Ed paa, at vedkommende er skyldig i Sagen til saa stort Beløb, som han tør give sin Ed paa. Paa det tredie Ting skal Nævningerne da træde til og enten sværge. ham sagesløs med Hensyn til de Koster eller til Tyv."

"How to progress in a case that rests on suspicion.

If a man knows who the thief is, and yet can not see his properties, then he shall sue that man that he presumes is guilty in the case, and let him trial at the thing, such as earlier said with robberies, and at the second thing he shall declare that the person is guilty for as large a sum as he dares to swear. On the third thing the jury shall step in, and swear him either thief or without cause"

I hope this helps confer a sort of general idea of how things would go about. A killed peasant would be working on his own farm and would have rights, if he worked on another mans farm, that man would be able to pursue the case as well as the peasants family, in some cases the kings man could step in and help investigate, and with a mysterious murder, probably the whole local community would be interested in finding the person who did it, but this is not something I have a source for.

This question has come up a number of times, and I usually like to drop in my post from a year ago on the subject of 'forensic criminal investigation' in the middle ages. Here it is in full, and I'm happy to answer any questions on it.

Medieval Popes and the Invention of Forensic Autopsy

One of the long-held, and incorrect, beliefs about the middle ages is that anatomy and dissection were forbidden, in particular under Christianity, and especially by the Catholic Church. Usually this is trotted out in defence of the conflict thesis: the argument that religion, and in particular the medieval Catholic Church, was opposed to science and therefore the progress of humanity.

When we look at evidence, we actually see that Innocent III ordered the first recorded autopsies since the classic period, paving the way for dissection science. Moreover, we can recognize these first forensic, legal activities as the direct forebears of today's investigative autopsies.

Two deaths and a Pope in 1209

Let's look at the stories of the first two juridical autopsies recorded in medieval history.

Our first anecdote is of a chaplain at the monastery Sancta Trinitatis of Maloleone near Bordeaux, who:

>having surprised a criminal in the act of stealing religious articles from the abbey chapel, struck the would-be felon with a heavy farm implement. The thief fled, despite his wounds, but was finally overtaken by parishioners who dispatched him with swords and clubs. The chaplain, fearing that the blow he had struck might have caused the death of the culprit even if the additional injuries had not been inflicted, related his story to the abbot.

The second is of a bishop at Siguenza near Toledo, who:

>disturbed by the rowdy behaviour of a number of his parishioners during Mass, ordered his canons to restore order about the altar. As their efforts proved to be insufficient to control the throng, he seized a cane and began to drive back the crowd by prodding some persons, and lightly striking others. The canons joined into this turbulent activity, and during the resulting melee, a young man was struck on the head. ^1

It would seem the young man was not affected by this blow: testimony states he thereafter ate and drank in taverns, he visited public baths, performed field labour. However, clearly something was amiss after this altercation with the bishop for although a month had passed, he was:

>advised to submit to an operation upon his injured head, [and] the victim allowed an old, unskilled physician to cut into both his skull and the flesh of his head. Four days after this operation the young man died, and though four physicians testified that the surgical procedure was ineptly performed, thus causing the youth's death, common talk charged the bishop with having fatally injured the young man with a blow from his cane. ^2

We have both of these stories from Regestorum sive epistolarum, a decretal of Pope Innocent III in 1209. Decretals were issuances of judgement on a variety of ecclesiastical matters, ranging from theology to canon law. In these cases of death involving clergy, both matters were submitted to the papacy for judgement and so come to us with the grounds for decision: the testimony of investigating physicians and surgeons.

In the case of the chaplain chasing the thief from his monastery, the matter was a question of establishing the chaplain's guilt according to Canon Law which stated that when several people are involved in a brawl and there is a death, the person who strikes the lethal blow was guilty of the homicide. Innocent III requested the testimony of expert physicians, and after receiving a report of their autopsy, it was declared that the chaplain did not strike the death blow [peritorum judicio medicorum talis percussio assereretur non fuisse lethalis].

The bishop of Siguenza sought to relieve public suspicion of his role in the death of the youth, suspicion which had jeopardized his position, and he appealed to the papacy for intervention. Again, the sworn testimony of surgeons and physicians was sought by Innocent III, and the pontiff upon examining the evidence sided with the testimony of these surgeons and physicians who declared the death a result of the botched surgery, and not the result of the bishop's blow [duo vero chirurgici et unus physicus jurari dixerunt quod non ex percussione sed indiscreta incisione obierat juvenis memoratus].

That Innocent III made central the function of law and legal process resulting in autopsies should not surprise us. Why not?

Origins in Medieval Law and Medicine

The scholastic effect of the medieval Christian Reconquista is a well-worn story by now, but worth restating briefly for its importance to our tale of how a pope came to lead the first recorded autopsies.

In the 11th century the Reconquista had taken scholastic Moslem cities such as Toledo, provoking the contact between Christian and Moslem scholastics, and in particular exposing Christian scholastics to unknown Greek and Roman works (in Arabic) as well as Arabic advancements. These covered science subjects like math, chemistry, physics, medicine, and also, perhaps most famously, Aristotelian philosophy. The path this knowledge followed into western European scholasticism are fairly complex and still being understood, but we can say that while the masters at the new universities of Oxford and Paris took up the massive theological implications of Aristotle’s works, the university cities of modern Northern Italy and Provence (Bologna, Milan, Montpellier) became centers of the resulting legal and medical scholasticism. ^3

By the middle of the 12th century, the university corridor of Bologna-Milan was the center and fount of legal scholasticism and training. It was here that Roman law was rediscovered, assembled, codified, and interpreted in the form that remains the basis of continental European (and other) legal systems today. Both this medieval civil law, the Corpus juris civilis, and Gratian's decretals-cum-canon law, Corpus juris canonici, find their blossoming here. Within this century-long development and codification, the canon documents of ecclesia (decretals and bulls) and the civil laws inherited from Justinian are interpolated and cross fertilized, borrowing processes and concepts from each other.^4

>During the twelfth century, in an age when disputes were still commonly settled by stone, iron, and flame, Europeans rushed to make use of new legal procedures provided by princes and popes, procedures that offered reason as an alternative to violence. Young men who had studied Roman or canon law were in demand everywhere, because they knew how to take testimony, weigh evidence, and put in writing their conclusions. ^5

Bologna university trained Lotario dei Conti of Segni to be canon lawyer after his time in Paris studying theology; and Lotario would become Innocent III. Bologna trained Lotario in the power of legal proceduralism and argumentation, of new legal ways to think which he used to great affect in re-asserting papal theocracy, of supremacy of the pope over secular rulers, and it provided many tools for formalizing, structuring, and enforcing Catholic orthodoxy ^6 .The greatest expression of this juridically-inclined Catholic orthodoxy was the 4th Lateran Council of 1215, perhaps the most famous act of Innocent's papacy and giving us the legalistic, rule-bound Catholicism we know today.

It should be no surprise to us as well that out of these legally-inclined, bureaucratizing generations of the papal curia should also come that other juridical expression of Catholicism: the medieval inquisition into heretical depravity.

Not coincidental for our story of autopsy and dissection, Bologna in 13th century also became the seat of medieval medical innovation and training. For much of the early and high middle ages of Western Europe, the works of the Roman Galen of Pergamon seemed to have fulfilled such need as there was for medical theory. The exposure to works outside of Galen via the Moslems, exposure to models of scientific inquiry, provoked in Bologna a rapid expansion of medical practices.

I would recommend reading Cynthia Neville's, Law, Land and People in Medieval Scotland. She gives a very straightforward and detailed description of legal processes and has done some excellent research on the topic so it would probably help answer you question even if it's region-specific.

We had a question on that some time ago and some answers include the time period you'd like to know about.

A similar question (in regards to England) was just posted two days ago

I see no answers about how it was in the Byzantine Empire (as usual to be honest). What do we know about what was happening there?

If you are at all interested in the legal systems for that time period in Ireland, check out Fergus Kelly's A Guide to Early Irish Law. And for the Welsh tradition, Dafydd Jenkins has an English translation with great notes and an intro for Hywel Dda.

I would pull some relevant bits for you myself, but I won't be home for a few. OP, if you're interested in those traditions but don't have access to a library etc, you can feel free to message me.

Peasants were an important resource, especially after the black plague (Nobles even stole each others peasants on occasion), so somebody would be pissed.

The way to see the system is as a patron-client thing. So there's no state, and hardly a society beyond the most immediate. Instead you're affiliated with a patron who may in turn have a patron etc. What you get is a lot of parallel "societies", where each is looking out for their own (think different guilds in a city). The patron gets revenue or whatever else you have to offer, and you get protection. Your patron cares if you get murdered.

Now jurisdiction is a whole problem in it's own right, with the church, the cities - even universities, claiming jurisdiction over their affiliates. Nobles also had varying degrees of jurisdiction in their domains, and of course over time the central authorities would try to, well, centralize authority.

As many punishments included fines or confiscation, people were actually competing over the jurisdiction (which, of course, is also a form of power in its own right).

The police would have been whatever armed force the relevant authority could muster, and I believe the investigation would have been mostly witness testimony