I've read hundreds of books on serial + mass murderers and this one is stumping me. I don't think there are any cases of a rather silly targeted "I'm coming for you Detective Biff McBifferson". Closest thing I can think of might be Eric Rudolph. He was hard core anti-gov't. He bombed an abortion clinic, but that wasn't the real target. He knew ATF and FBI would cordon off the area, and he had a second bomb set to go off later just outside this area, the goal being to kill the investigators themselves. It's a tactic that the IRA used to use. In that instance it didn't work because due to blind luck someone parked a big car in between the bomb and the investigators, and it deflected most of the blast.

/u/abanks47 mentioned organized crime downthread. Though it might not fit the Hollywood "Hannibal Lecter" type of serial killer, many organized crime figures meet the traditional "3 or more murders" definition of serial killers.

So if you're prepared to accept organized crime figures as serial killers, a good place to start is the Sicilian Mafia. There was a notable spree of killings in the late 1970s and early 1980s of investigative and judicial figures.

It kicked off with the murder of Boris Giuliano, an Italian police officer who was killed after uncovering a major money-laundering operation by the Sicilian Mafia (the money was from heroin trafficking to America). The man convicted of killing Giuliano was Leoluca Bagarella, who certainly meets the "3 victims" definition of a serial killer.

Unfortunately, Giuliano's death was not the end of Mafia murders of people working in the criminal justice system. Two months after Giuliano was murdered, the crusading Anti-Mafia judge and former member of parliament Cesare Terranova was murdered along with his bodyguard, who was a police officer.

If you include 'conspiracy to commit murder' in the body count required to become a serial killer, then a good number of the Sicilian Mafia's leadership during the late 70s would qualify as answers to your question.

There was also a second major anti-government burst of violence from the Sicilian Mafia in the early 1990s, and the two most prominent victims were both judges: Giovanni Falcone and Paolo Borsellino.

Both Falcone and Borsellino had been prosecuting magistrates who went after Mafia figures. As the Italian justice system works differently than the common law adversarial system, these prosecuting magistrates occupy a legal role that combines investigative and adjudicative functions.

Though I'm less familiar with it, the Colombian (and probably Mexican) organized crime groups can claim similar intimidation killings against people working in the legal system. The Medellin Cartel during Pablo Escobar's leadership made a frequent practice of killing police, journalists, judges, and even members of government.

Closest I know of is Ridgway ("The Green River Killer) going to lectures about the hunt for himself by author Ann Rule. Rule was a former cop turned true crime author and had good contacts in the police regarding the hunt, and Ridgway supposedly came to them to try to see how close they were to identifying him, and even asking questions.

As an aside, Rule previously shared a cubicle with Ted Bundy before his cross-country killing spree, when they both worked at a nonprofit.

Green River, Running Red by true-crime author and former police officer Ann Rule (September 27, 2005)

Alain Lamare. I have trouble getting sources in English, but he's kinda famous in France as "le tueur de l'Oise" / the Oise killer. A movie has been made about him last year, La prochaine fois je viserai le coeur.

It happened in 1978 and 1979. He's not really a serial-killer though. Or is he ? He tried though. He only killed once, but he tried numerous times. He boobytrapped stolen cars that blew up to cops face (not killing them), he attacked girls but only killed one, paralysing another, injuring a third, and the police tried to catch him for almost a year before they succeeded. During his crazy rampage, he sent letters to the police anonymously to say he did it, and said he would do it again. His letters raised suspicion because of how it was written: it was written as written by a cop. That helped catching him, event though the hierarchy first denied a cop could do it. He was indeed a cop ( a gendarme actually. A military, but with police duty. )

So Alain Lamare was investigating on his own case. So he knew what the cops plan was, and thought ahead.

But in the end he got caught. And, the thing is, regarding this post's topic, he planned to attack his fellow gendarmes. Thankfully, they caught him before. They found a military arsenal in his home, and notebooks, where he planned to do more damage by attacking his superiors, and also at least the wife of one them. He also said it to them: "You're lucky you got me, because i would have gotten you".

So a kinda serial-killer, or at least a wannabe-one (a big weirdo anyway), who was actually a cop nagging the police and investigating on his own crimes at the same time, planned to kill his "friends" investigating on his case.

There is not a lot of source as I said. Wikipedia is rather short - and it happened almost 40 years ago. It made some fuzz at the time but it's kinda forgotten (with the exception of the movie released last year). Here are a few in french, though.

An article released last year about a guy Lamare planned to kill : I was on the list of the Oise killer : http://www.limpartial-andelys.fr/2014/11/22/les-andelys-%C2%AB-jetais-sur-la-liste-du-tueur-de-loise-%C2%BB/ A global overview on Wikipedia : http://fr.wikipedia.org/wiki/Affaire_Alain_Lamare A well done documentary, very detailed : https://www.youtube.com/watch?v=_F8tr5TG1S4 And the movie La prochaine fois je viserai le coeur. I don't know what its worth, haven't seen it.

This was probably (almost certainly) more a case of accidental overreach, but the second-to-last victim of Charlie Starkweather and Caril Anne Fugate, the industrialist C. Lauer Ward, was a friend of the governor of Nebraska, and had actually been meeting with the governor when Starkweather and Fugate broke into his home and killed his wife and housekeeper.

Then he came home, found them, and was killed himself.

After Ward's body (and the bodies of his wife and maid) were found, Governor Anderson called the National Guard to help the Lincoln police, county sheriff and state troopers capture the killers.

Now, Lauer wasn't targeted because he was investigating the killers, but he was definitely a prominent citizen, which was probably why he attracted the killers. I mean, I think they just went to the nicest house in the nicest part of town without knowing who lived there.

Oops, buddies with the governor.

I'm also certain that /u/meninthemirror/ is right about cartel violence, and that in certain places in Mexico, the local law enforcement was outright replaced by members of the cartel.

Anecdotally, I heard about this firsthand while driving through Sinaloa in the 1990s. There's plenty on the record about the rise of the Zetas, though... most, however, within the 20-year exclusion rule. Much of the membership of the Zetas consisted/consists of cops and soldiers hired by the Gulf cartel.

I'm not sure if "mercenary" counts as "serial killer" though. Not the same psychological profile. For one thing, they act under orders....

I'm not entirely sure if this comment is allowed but wanted to pitch in and say maybe the folks over at /r/UnresolvedMysteries might know a bit about this. They're usually very nice and well-read in all sorts of criminal history, including serial killers. I apologise if I'm breaking a rule and if I am, I'll delete the comment and just PM the OP instead.