A Black Hawk helicopter went down Wednesday in northern Iraq, killing all 14 U.S. soldiers aboard, the military said, the deadliest crash since January 2005.

The military said initial indications showed the UH-60 helicopter experienced a mechanical problem and was not brought down by hostile fire, but the cause of the crash was still under investigation.

It was one of two helicopters on a nighttime operation. The four crew members and 10 passengers who perished were assigned to Task Force Lightning, the military said. It did not release identities pending notification of relatives.

The U.S. military relies heavily on helicopters to avoid the threat of ambushes and roadside bombs — the deadliest weapon in the militants' arsenal — and dozens have crashed in accidents or been shot down.

The deadliest crash occurred on Jan. 26, 2005, when a CH-53 Sea Stallion transport helicopter went down in a sandstorm in western Iraq, killing 31 U.S. troops.

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Wednesday's deaths raised to at least 3,721 members of the U.S. military who have died since the Iraq war started in March 2003, according to an Associated Press count.

Elsewhere in northern Iraq, a homicide truck bomber targeted a police agency in northern Iraq, killing at least 27 people and wounding 65, police and hospital officials said.

The attack occurred just before noon in Beiji, 155 miles north of Baghdad, and many of the casualties were civilians, according to the officials.

Brig. Gen. Kevin Bergner, a U.S. military spokesman in Baghdad, said the attack bore all the hallmarks of al-Qaida in Iraq, including the use of a suicide bomber and the high number of civilian casualties.

"It appears to be something that is consistent with an al-Qaida-related attack," he told AP Radio in an interview.

Iraqi police and soldiers have frequently been targeted by militants seeking to disrupt U.S.-led efforts to enable the forces to take over their own security so foreign troops can go home.

Those killed Wednesday included 18 policemen and nine civilians, while 20 officers and 45 civilians were wounded, the officials said.

A bomb and small-arms attack against a security post shared by police and U.S. paratroopers also killed 13 Iraqi officers in Beiji in late June.

Jassim Saleh, 41, who lives about 500 yards from the blast site, said he saw an explosives-laden truck carrying stones strike the police station.

"It was a horrible scene. I can't describe it," he said. "The bodies were scattered everywhere. I was injured in my hand and a leg, but I took three wounded people to the hospital in my car."

Police said nine policemen and 10 civilians were killed, while 21 civilians and five officers were wounded.

A roadside bomb also targeted a police patrol in the center of Tikrit, Saddam Hussein's hometown 80 miles north of Baghdad, killing one officer and wounding another, along with two civilians, authorities said.

Complete coverage is available in FOXNews.com's Iraq Center.