In the early morning hours of March 16, 1968, during Charlie Company, 1st Battalion, 20th Infantry Regiment, Americal Division's assault on a hamlet known on U.S. military maps as My Lai 4, Colburn's OH-23 helicopter surprisingly encountered no enemy fire while hovering over this suspected headquarters of the Viet Cong 48th battalion. Spotting two possible Viet Cong suspects, Thompson forced the Vietnamese men to surrender and flew them off to the rear for tactical interrogation. He also marked the location of several wounded Vietnamese with a green smoke marker, a signal that they needed help.