[Stalin] put Soviet POWs through 'filtration' camps after their liberation from German captivity. An astonishing 2,775,700 former soldiers in the Red Army were subject to interrogation upon repatriation, and about half of them landed up in a labour camp.
Source. (second and third sentences)
Wiki entry for "filtration" camps.
I am particularly interested in learning about the experiences of former Russian soldiers who went through one of these "filtration" camps. I cannot imagine what it must have been like to have been captured as a POW, survive, and then be accused of treason to the Soviet government when you return. Are there any eyewitness accounts or other narratives about these filtration camps?
The Russian site "Я Помню" - I Remember - (https://iremember.ru/) has a number of accounts by Red Army veterans which describe the camps. Here's one by Grigory Davidovich Vodyansky:
A: From the first minutes on our home soil we were treated like traitors
Q: How was the state examination at the Murom filtration camp?
A: We were lodged in barracks on empty wooden plank beds.
There were no physical punishments or such applied to us. But all the time we heard threats from the guards and the investigators. During the first days the “suspicious” individuals were separated, as well as the officers from the ranks.
The investigators called in people one by one for a thorough interrogation. After about a week my turn came. The first thing I heard from the investigator was: “Are you Grigory D. Gurin? Take a seat, traitor! Tell us where and when and how you surrendered to the enemy?”
I replied: “Firstly, I'll stand. And my surname isn’t Gurin but Vodiansky”. His reaction was promt. He jumped up and shouted right into my face: “Are you saying that you're a Jew? Then tell me how you, a Jew, managed to survive in a German concentration camp?” Then I presented my detailed account naming all the units I served with in the RKKA, objectives of our Airborne Brigade, the circumstances of my surrender (I was injured) and that there are two living witnesses to it.
When I ended my narration, the investigator was silent for a while and then said: “Well, you're dismissed… for now… expect another interrogation session.” After two weeks the guards called me in again. Unlike the first session this one was friendly. The interrogator offered me a chair. Then he asked me, severely: “Why did you not let your parents know that you were back and healthy?” I said that I did not know their whereabouts. They were evacuated to Cheliabinsk, but that was two years ago. The investigator replied: “We pulled some strings and found out that your parents live in Ukraine, in the town of Kherson.”
He gave me the address and told me to get in touch with them. At the end of our talk he said: “Expect to be called in again, but next time it will be by other people.” The next time I was called in by the people who arrange job placements for ex-POWs in the civilian sector. I received a temporary ID card, which stated that I passed filtration and was cleared, that I was a Soviet citizen, and had the right to vote.
I was sent to the town of Rostov, Yaroslavl, to work on a lumber enterprise. On the 10th of June 1946 I left that workplace and headed to the town of Kherson as a free man. But after arriving and as soon as I registered I was again called for a talk at the local State Security Department office. These continued for several months.
If you're interested in more academic information I can do a roundup on the topic of filtration? Russian literature on the topic is of course vast, and I think Mark Edele and Vanessa Voison have written about it too.