The Fairfax County police officer who disappeared during a diving training exercise Tuesday is believed to be dead, police said Thursday as they continued to search for his body.

Second Lt. Frank Stecco, 43, was a volunteer role-player in a helicopter water rescue exercise in a Potomac River bay when he disappeared around 2:50 p.m. Tuesday. Extensive efforts from multiple agencies, including police from across the region, the Coast Guard and the FBI, have been unable to locate him.