M: Eine Stadt Sucht Einen Mörder (1931)

There have been a rash of child abductions and murders in Berlin. The murderer lures the children into his confidence by candy and other such child friendly items. Everyone is on edge because the murderer has not been caught. The most substantial pieces of evidence the police have are hand written letters by the murderer which he sent to the newspaper for publication. Unknown even to himself, a blind beggar, who sold the murderer a balloon for one of the child victims, may have key information as to the murderer's identity. The murder squad's work is made even more difficult with the large number of tips they receive from the paranoid public, who are quick to accuse anyone of suspicious activity solely for their own peace of mind that someone - anyone - is apprehended for the heinous crimes. Conversely, many want to take the case into their own hands, including the town's leading criminals since the increased police presence has placed a strain on their ability to conduct criminal activity. Although they both have the same end goal of capturing the murderer, the police and the criminals seem to be working at cross purposes, which may provide an edge to the murderer in getting away.

IMDb: https://www.imdb.com/title/tt0022100