The Taliban has claimed responsibility for the crash, and a US military official has apparently confirmed that the helicopter was indeed brought down by hostile fire:

Insurgents shot down a NATO Chinook helicopter during an overnight operation in eastern Afghanistan, killing at least 37 people on board, a coalition military official said on Saturday. It was believed to be the deadliest helicopter crash in the nearly decade-long war, punctuating a surge of violence across the country even as American and NATO forces begin a modest drawdown of troops.

Afghan military officials put the death toll at 38, including 31 Americans and 7 Afghan commandos.

[...]

The helicopter was hit by a rocket-propelled grenade in the Tangi Valley of the Wardak Province just west of Kabul, the coalition official said. The Taliban claimed credit for the attack.

If confirmed, Saturday's crash would be the deadliest day for American forces since the war began. A NATO spokesman, Capt. Justin Brockhoff of the United States Air Force, confirmed the crash but could provide no further information, including what caused the crash or whether there were casualties.

There were conflicting accounts about when the helicopter went down. A spokesman for the Taliban, Zabiullah Mujahid, said insurgents shot down the helicopter around 11 p.m. Friday as it was starting an operation on a house where the militants were gathering in the Tangi Joyee region of the district of Saidabad in the eastern part of the province. Eight militants were killed in the fight, which continued after the helicopter fell, Mr. Mujahid said.

"The fresh reports from the site tells us that there are still Americans doing search operations for the bodies and pieces of the helicopter are on the ground," he said.

The crash represents the single greatest one day loss of life in the war. It surpasses the 19 dead caused when the Taliban shot down a helicopter full of Navy SEALs in 2005.