The phrase "unknown gunmen" refers to individuals who are involved in some violent or criminal activity but their identities are not known. Full definition
The Nigerian Air Force, NAF, headquarters has said that its helicopter landing site, Helipad, in Yenagoa, Bayelsa State capital was attacked by unknown gunmen on Sunday... (dailypost.ng)
In 2010, precisely September 14, the head of the Commission's Forensic Unit, Abdullahi Muazu was shot and killed by unknown gunmen in Kaduna. (nigeriapoliticsonline.com)
The Nigerian Airforce headquarters has said that its helicopter landing facility (Helipad) in Yenagoa, Bayelsa State was on Sunday attacked by unknown gunmen on Sunday / The Airforce said that its personnel succeeded in repeling the attack even as it lost one airman during the shootout. (securitykingng.org)