( CNN ) -- The Pentagon urgently investigated whether six American troops killed in Afghanistan on Tuesday may have died from enemy fire after their helicopter crashed , rather than from the crash itself , two senior U.S. officials said . `` We do not know how those killed were killed , '' one of the officials told CNN . `` It 's unclear if it was the crash or contact with the enemy after the helo came down . '' One person survived . 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 . On Tuesday , the Pentagon said initial reporting indicated that six members of the NATO-led International Security Assistance Force were killed when their Black Hawk helicopter crashed . Military officials cited potential mechanical failure and said at the time there was no indication of enemy activity in the area where NATO forces move around frequently . Later in the day , questions surfaced about whether the troops may have actually survived the crash and then came under mortar fire . `` We believe there was some sort of enemy engagement once the helicopter crashed , '' one of the officials said . Both officials declined to be named because of uncertainty over what occurred . They emphasized strongly there had been no conclusion about the report of enemy fire . Crew members of a second helicopter flying in the area would be interviewed . Investigators also would look at the wreckage and conduct autopsies for clues . Names of those killed were not released . Interactive : Coalition troops killed in Afghanistan , Iraq . Tuesday 's incident raised to 129 the number of U.S. troops who died in 2013 while supporting Operation Enduring Freedom . The vast majority were lost in Afghanistan . CNN 's Barbara Starr and Ric Ward contributed to this report .
U.S. forces were in helicopter crash in southern Afghanistan . Pentagon received report that troops came under mortar fire after crash . Officials say no conclusions reached yet in crash initially blamed on mechanical failure . Tuesday 's toll worst single-day for U.S. troops since August 2012 .