The police suspect a lone gunman committed the crime.
lone gunman
idiomAn individual person who acts on his or her own initiative, without partners, especially one who has sole responsibility for doing something questionable, confidential, or iniquitous.
Origin
disputed originFrom the Warren Commission's lone gunman theory, that the assassination of U.S. President John F. Kennedy was the work of a single individual, subsequently applied to numerous other political assassinations and shootings.
Investigators initially searched for a group of suspects, but evidence soon suggested the act was the work of a lone gunman.
Historians continue to debate whether the tragic event was a coordinated conspiracy or the isolated action of a lone gunman acting entirely on his own accord.