Six American troops died when a military helicopter crashed in Southern Afghanistan on Tuesday, according to U.S. officials.
The cause of the crash is still unclear, CNN reports, but military authorities are now investigating a report that enemy mortar fire hit the chopper while it was still on the ground. Earlier reports suggested that the engine failed.
One person on board survived the crash.
The incident is the deadliest event for American forces in Afghanistan since the summer of 2012, when a helicopter crash killed seven Americans. With Tuesday’s tragedy, a total of 129 American troops have been killed supporting the U.S. mission in Afghanistan. Barring other major catastrophes, however, 2013 will likely be the least deadly year for U.S. troops in the country since 2007.