Five soldiers were injured and one was missing after the Saturday crash.
The chief military spokesman in Baghdad, Col. William M. Darley, said the cause of the helicopter crash "will be under intense investigation today."
The two Black Hawks, which belonged to the 101st Airborne Division, went down in residential neighborhoods of Mosul, Iraq's third-largest city.
The spokesman for the 101st Airborne, Maj. Trey Cate, said the military was "trying to figure out what happened.... We are going to do a thorough investigation because if this either involved ground fire or it was safety-related, then ... we're going to make sure we take precautions so it won't happen again."
A statement by the U.S. command said one helicopter was carrying a quick reaction force and the other ferried soldiers on a transport mission in northern Iraq. Cate said the quick response team was on its way to investigate a shooting incident in which a U.S. soldier was injured.
The statement did not give the cause of the crash.
CBS News Reporter Charles D'Agata said a U.S. military official at the scene conceded that one of the choppers was hit by a rocket-propelled grenade. Iraqi eyewitnesses say that chopper then collided with the second Black Hawk, bringing both down.
If it was an attack that brought them down, it would bring to 39 the number of coalition troops who have died in helicopter attacks in the past three weeks.
An Iraqi policeman in Mosul, however, 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 told The Associated Press 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.
By Sunday morning, soldiers were busy clearing the rubble at the two crash sites. Two cranes were being towed in to help the clean up operation.
Cate said the military was still removing bodies of soldiers from the site.
According to Cate, one helicopter had 12 soldiers on board; seven were killed and five injured. The other had 10 aboard; all were killed.
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.