The usual suspects
The people habitually suspected or arrested in response to a crime. The phrase is usually used in regard to scapegoats rather than actual perpetrators of the crime in question.
This expression has a specific and unambiguous origin. It was spoken by Captain Louis Renault, the French prefect of police, played by Claude Rains in the 1942 U.S. film Casablanca. The context was a scene in which the Nazi, Major Strasser is shot by Humphrey Bogart's character, Rick Blaine. Renault was a witness to the shooting but saves Rick's life by telling the investigating police to "round up the usual suspects". The film then ends with the famous exit line:
"Louis, I think this is the beginning of a beautiful friendship."
The screenplay credits list Julius J. Epstein, Philip G. Epstein and Howard Koch as the film's writers.
'The usual suspects' is now as commonly used as other lines from the film that were spoken by Bogart and which were much more quickly taken into the public consciousness:
Play it once, Sam. For old times' sake.
Here's looking at you, kid.
Of all the gin joints in all the towns in all the world, she walks into mine.
The phrase appears a few times in print from the 1950s onward but didn't become commonplace until the 1990s. That may have been influenced by the eponymous 1995 film noir, directed by Brian Singer.