The U.S. military on Sunday was investigating whether attacks from the ground caused the crash of two U.S. Black Hawk helicopters (search) that killed 17 American soldiers.

The chief military spokesman in Baghdad, Col. William M. Darley, said the cause of the crash "will be under intense investigation today."

The exact cause of the crash is not yet known.

Violence in the area also continued on Sunday. A roadside bomb exploded in Mosul (search), hitting an Iraqi minibus, slightly injuring four people, Iraqi police said. There were no U.S. troops in the area of the blast.

Soldiers used cranes to clear rubble Sunday and removed all the bodies from the residential neighborhoods where the two choppers crashed Saturday night in Mosul, Iraq's third largest city. The crash caused the worst single loss of U.S. life since the start of Operation Iraqi Freedom (search), putting the total number of U.S. deaths since the March invasion at 417.

All the casualties were from the 101st Airborne Division, which controls northern Iraq. Five soldiers were injured.

"We don't yet know exactly what happened," the top U.S. administrator in Iraq, L. Paul Bremer (search), said in a television interview Sunday. "There had been reports there may have been some ground fire but frankly I'm waiting until the military finishes its investigation. Obviously a great tragedy for all of the people."

One helicopter was carrying a quick reaction force and the other ferried soldiers on a transport mission in northern Iraq. One helicopter had 12 soldiers on board; seven were killed and five injured. The other had 10 aboard; all were killed. Initial reports that one soldier was missing were incorrect, a military spokesman said.

Maj. Trey Cate, a spokesman for the 101st Airborne, said the quick response team was on its way to investigate a shooting incident in which a U.S. soldier was injured.

An Iraqi policeman in Mosul said at least one of the Black Hawks was hit by ground fire.

"They hit it with a missile," said policeman Saddam Abdel Sattar. "I was in the army, I know these things."

Another witness, Yousra Khedr said she saw one helicopter above her house before hearing the sound of a loud boom.

"I saw the sky light up, it was like thunder and lightning," she said, adding that after the initial boom she heard gunfire in the area but could not say where it came from.

One soldier at the scene said he heard that one of the helicopters was hit by a rocket-propelled grenade before it crashed. A U.S. military spokesman said such reports were "at best speculative."

Another witness said he heard gunfire on the ground before the crashed.

"The Black Hawks were in the air and there was shooting (on the ground). It was dark and one slammed into the other," said an Iraqi Civil Defense Corps soldier who identified himself only as Mahmoud.

Before the crash, the U.S. military's deadliest incident was the downing of a Chinook transport helicopter on Nov. 2 that killed 16 soldiers. A Black Hawk was also shot down on Nov. 7, killing all six soldiers on board.

There were days early in the war in which more soldiers died, but they were spread over several attacks or accidents.

Defense Secretary Donald Rumsfeld, traveling in Asia, said the United States continues to plan to rotate a new contingent of troops into Iraq next year, with no final pullout date set yet. Accelerating the political process toward Iraqi self-rule will not affect military planning, he said.

In other news, the Iraqi Governing Council on Saturday endorsed a U.S. plan that would create a provisional government by June. The transfer of power would provide Washington with an "exit strategy" in the face of escalating guerrilla warfare.

The plan reflected Washington's desire to speed up the handover of power as attacks against American occupation forces grow more sophisticated and deadly. The Bush administration dropped its insistence that a constitution be drawn up and elections held before the transfer takes places.

However, one of the 24 members of the council warned that "execution of the plan won't be easy" without improvement in the security situation and a revival of Iraq's economy.

And Defense Secretary Donald H. Rumsfeld cautioned Sunday that the accelerated plan for restoring Iraqi self-rule does not mean U.S. troops will withdraw anytime soon.

"The timetable or the way ahead that the (Iraqi) Governing Council has been describing relates to the governance aspects of the country and not to the security aspects," he said. "That's on a separate track

The council announced a set of deadlines that would give Iraq a provisional national assembly by May, a transitional administration with full sovereign powers in June and an elected government before the end of 2005.

The U.S. military occupation would then formally end, although American forces are expected to remain in Iraq under a new arrangement to be worked out with the Iraqis.

Until a constitution is drafted and adopted, a basic law will be promulgated by the Governing Council and take effect in February.

The law would establish a democratic and federal state that "respects the Islamic identity of the majority of the Iraqi people with the guarantee of the right of other religions and sects."

It will enshrine respect for human rights and ensure equality of members of the country's diverse religious and ethnic groups.

The Associated Press contributed to this report.