Bug with Cohen's third photo (BioShock)

After I insert a photo of Cobb (the third in a row) into Cohen's sculpture, he sends mutants. When I finish with all of them, the music continues to play and the tasks include the same "Kill all Cohen's mutants". I go around the whole level, but I can't find the missing ones. Has anyone encountered this bug? Saved before this, now I can not do anything about it .... (((