By Barbara Starr
CNN has learned the U.S. military has concluded that Taliban insurgents brought down a Black Hawk helicopter in Afghanistan on December 17, killing six American troops on board.
It was the single deadliest day for U.S. forces in Afghanistan since a helicopter wreck killed seven Americans and four Afghans in August 2012.
That conclusion follows weeks of confusion and conflicting intelligence reports about what was behind the crash.
Three U.S. officials tell CNN that an enemy-generated explosion brought down the helicopter in Zabul province.
Two officials say it's possible the low-flying helicopter set off an improved explosive device on the ground. If that is the case, it could pose new challenges for U.S. helicopter operations in the war zone, the officials said. They all declined to be identified because of the sensitive nature of the information.
In response to questions from CNN, the International Security Assistance Force responded via e-mail, saying "the families of the soldiers killed in the December 17 helicopter crash have been notified that enemy action caused the crash and loss of life. Although the investigation is not yet complete, we informed the families at this time out of respect so they know how their loved ones died. The investigation is ongoing, and more details will be provided when the investigation is complete."
The assessment is based on a variety of evidence collected. The crew of another helicopter flying nearby reported seeing a flash but did not see enemy forces on the ground at the time. One surviving crew member has also been interviewed. And investigators have also closely analyzed debris on the ground and damage to the aircraft.
The incident generated extensive confusion at the highest levels of the military, leading to unusual sensitivity about discussing it. Initially, military officials cited potential mechanical failure and said there was no evidence of enemy activity. Then officials said reports had surfaced about whether the troops may have come under mortar fire after surviving the crash. Then some officials said those reports were not accurate, either.