HONOLULU (AP) – An Air Force B-52 bomber crashed off Guam on Monday morning, killing at least two airmen and leading to an ocean search the remaining four crew members, the military said.

Six vessels, three helicopters, two F-15 fighter jets and a B-52 bomber were involved in the search, which had covered about 70 square miles of the Pacific, said Coast Guard spokeswoman Lt. Elizabeth Buendia.

“We have an active search that’s going to go on throughout the night,” she said. The Navy, Coast Guard, Air Force and local fire and police departments were involved.

Maj. Stuart Upton, a Pentagon spokesman, said the aircraft was unarmed.

The B-52 bomber was en route from Guam’s Andersen Air Force Base to conduct a flyover in a parade on another part of the island when it crashed around 9:45 a.m. Monday.

The Liberation Day parade celebrates the day the U.S. military arrived on Guam to retake control of the island from Japan.

The Air Force said a board of officers will investigate the accident.

The accident is the second for the Air Force this year on Guam, a U.S. territory 3,700 miles southwest of Hawaii.

In February, a B-2 crashed at Andersen shortly after takeoff in the first-ever crash of a stealth bomber. Both pilots ejected safely. The military estimated the cost of the loss of the aircraft at $1.4 billion.

The plane that crashed Monday was based at Barksdale Air Force Base in Louisiana and deployed to Guam as part of the Department of Defense’s continuous bomber presence mission in the Pacific. The Air Force has been rotating B-1, B-2 and B-52 bombers through Guam since 2004 to boost the U.S. security presence in the Asia-Pacific region while other U.S. forces in the area have been sent to the Middle East.

Master Sgt. Cindy Dorfner, a spokeswoman for the Air Combat Command in Langley Air Force Base, Va., said the last crash involving a B-52 was on June 24, 1994 in Spokane, Wash. The bomber was practicing touch-and-go landings before an air show at Fairchild Air Force Base when it plunged to the ground and exploded, killing all four on board.

The Air Force has 93 B-52 bombers remaining in its fleet.

The B-52 is a long-range, heavy bomber that can refuel in mid air. Since the 159-foot-long bomber was first placed into service in 1955, it has been used for a wide range of missions from attacks to ocean surveillance. Two B-52s, in two hours, can monitor 140,000 square miles of ocean surface.

According to the Air Force’s Web site, the B-52 Stratofortress has been the backbone of the manned strategic bomber force for the United States for more than four decades. It is capable of dropping or launching the widest array of weapons in the U.S. inventory, including cluster bombs and precision guided missiles.