Four U.S. soldiers were killed when their Black Hawk helicopter crashed during a training mission in eastern Afghanistan on Thursday, 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 Qaeda 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.

An Associated Press reporter in Afghanistan who attempted to reach the scene of the crash was turned back by Afghan security. U.S. military authorities at Bagram referred all questions to Central Command.

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

Officials initially said the helicopter was the utility version of the Black Hawk known as the UH-60, but later they said it was the multi-mission variety known as the MH-60, which is used by Army special operations forces for long-range, low-level penetration of hostile territory at night.

The Air Force flies yet another version known as the MH-60G Pave Hawk, used for special operations missions.

The helicopters are manufactured by Sikorsky Aircraft Corp., which also makes an export version called the S-70A Black Hawk.

The UH-60 Black Hawk was first fielded in 1978 and made its combat debut in the October 1983 invasion of Grenada. The Army's MH-60 special operations version was first fielded in 1994.

The last previous 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, and at least 11 other troops have been injured.

On Oct. 19, 2001, Army Ranger Pfc. Kristofor Stonesifer, 28, and Spc. John J. Edmunds, 20, were killed in the crash of a Black Hawk in Pakistan.