Washington ? Three U.S. soldiers were killed when their helicopter crashed into the Tigris River in northern Iraq, Pentagon officials said Friday.

The UH-60 Black Hawk helicopter crashed about 8 p.m. local time (11 am. CDT) near Samarra, between Baghdad and Tikrit, the officials said. The only other soldier on the helicopter was injured.

Preliminary reports indicated the crash was an accident, a senior Pentagon official said.

The three deaths bring the number of U.S. troops killed in the Iraq war to 145.

The Black Hawk is the Army’s main troop transport helicopter. A crew of four usually flies the Black Hawk, which can carry up to 11 more soldiers.

The last Black Hawk to crash in Iraq was shot down by Iraqis near Karbala April 2, killing six.

A Black Hawk crashed in March in a remote, wooded area of Fort Drum, N.Y., during a training exercise, killing 11 of the 13 soldiers aboard.

In February, a Black Hawk crashed during night training in the Kuwaiti desert, killing all four crew members.

In other developments:

l Defense Secretary Donald H. Rumsfeld said it was not possible to know how long U.S. forces will have to remain in Iraq and suggested that stabilizing the newly liberated country could take longer than a year. Rumsfeld told a news conference a one-year timeline attached to the presence of U.S. and British forces in Iraq was probably “just a review period” in the overall postwar plan.

l The U.S. Army surrounded camps of an Iranian opposition group, Mujahedeen Khalq, in eastern Iraq on Friday, pointing tanks at its sentinels and demanding it lay down arms or “be destroyed.” U.S. troops were prepared for combat but were negotiating with members of the group Friday.

l U.S. officials said they had located the main vaults of the nation’s central bank in Baghdad — and found they were apparently not looted. They said the safes contain U.S. dollars, Iraqi dinars, gold and items from the museum put there for safekeeping.

l The commander of the American weapons hunters in Iraq says he’s certain the U.S. invasion has ended a program capable of producing Iraqi chemical and biological weapons. But Col. Richard R. McPhee says his teams have found no such weapons thus far.

l Elizabeth Neuffer, an award-winning reporter for The Boston Globe, died in a car accident in Iraq while on assignment covering the aftermath of the war, the newspaper said.

l Coalition forces are investigating a mass grave near the southern Iraqi city of Samawah that may hold the remains of Kuwaiti prisoners from the 1991 Persian Gulf War, officials said.

l Eighty people were rounded up in a northern Iraqi village after American forces were told they might find four local leaders of Saddam Hussein’s Baath Party there, the U.S. military said.