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Blackhawk helicopter that crashed in southern Afghanistan last month, killing six U.S. soldiers, was brought down by an enemy explosion, not gunfire or mechanical failure, U.S. military officials said Thursday.

The UH-60 helicopter crashed in the Shahjoi district of Afghanistan's Zabul province on Dec. 17. Initial reports indicated it came under heavy enemy gunfire once it was on the ground, but later reports contradicted that, implying there was no enemy contact.

By the time a rescue team was deployed to the scene of the crash, six of the seven crew members had been killed, with one surviving with serious injuries. The soldiers were Americans serving with the NATO International Security Assistance Force.

According to U.S. military officials, two U.S. Army Blackhawk helicopters were flying in tandem when one suddenly dropped from the sky. The cause was under investigation, NATO said at the time.

On Thursday, U.S. military officials told NBC News the helicopter was in a low-hover position when a blast from the ground below caused it to crash. The helicopter was not shot down by enemy fire, and apparently no enemy forces were on the ground in the vicinity either before or after the fatal crash.

At the time of the incident, U.S. military officials in Afghanistan strongly denied that the helicopter was brought down by "enemy fire," but did not divulge the crash was the result of "enemy action."

The incident was the single deadliest for American forces in Afghanistan in more than a year. The investigation into how exactly the blast occurred is ongoing.

Aircraft crashes are not uncommon in mountainous Afghanistan. In August 2011, 25 U.S. special operations forces were among 38 killed when the Taliban shot down a transport helicopter.

About 43,000 American troops remain in Afghanistan, and about 80,000 total ISAF troops.

A pending security agreement with the Afghan government could drop the number of Americans to under 10,000 by the end of 2014. That agreement, however, would commit U.S. forces to another 10 years in Afghanistan.