Washington ? Four U.S. soldiers were killed when their Black Hawk helicopter crashed during a training mission Thursday in eastern Afghanistan, U.S. officials said. The cause was being investigated.

It was the deadliest day for the American military in Afghanistan since March 4, 2002, when seven soldiers were killed and 11 wounded at the outset of an offensive against Taliban and al-Qaida remnant forces.

The Black Hawk, with two pilots and two crew members aboard, crashed several miles east of Bagram air base in an area known as the East Training Range, said Jim Wilkinson, director of strategic communications at Central Command headquarters in Tampa, Fla.

Wilkinson said there were no indications of hostile fire. He said it appeared to be an accident, but no other details were available.

Details on the victims, including their names and Army unit affiliations, were not immediately released.

Bagram is the main base of U.S. military operations in Afghanistan.

The last American killed in Afghanistan was Sgt. Steven Checo, 22, a member of the Army’s 82nd Airborne. He was shot Dec. 21, 2002, while on a nighttime operation in the eastern province of Paktika, near the border of Pakistan.

Since U.S. military action in Afghanistan began in October 2001, at least five U.S. helicopters have crashed or had hard landings that have injured or killed troops. Two Army Rangers and two Marines have been killed.

U.S. military ground crew members perform maintenance work on a Black Hawk helicopter at the U.S. military air base in Bagram, Afghanistan. A different U.S. Army Black Hawk helicopter with several Americans aboard crashed Thursday near the air base, killing four soldiers, officials said.