The phrase "lone gunman" refers to an individual who acts alone when carrying out a violent or illegal action. It often describes someone who commits a crime or an assassination without assistance or involvement from others. Full definition
In the event of such an attack, with lone gunmen indiscriminately shooting civilians in crowded places, police armed response units would be tasked with containing the terrorists. (politics.co.uk)
This is one reason many people were uncomfortable with the idea that President John F. Kennedy was the victim of a deranged lone gunman and found it easier to accept the theory that he was the victim of a large - scale conspiracy. (scientificamerican.com)
In November, a lone gunman interrupted Sunday services with his Ruger AR assault rifle, murdering more than two dozen people — including the pastor's 14 - year - old daughter — and wounding 20 others. (fortune.com)