U.S. troops were in helicopter operations in southern Iraq . Pentagon received report that troops came under mortar fire after crash . Pentagon say no injuries reached yet in crash initially blamed on mechanical failure . Both officials declined to be named because of uncertainty over what occurred . Tuesday 's toll worst single - day for U.S. troops since January 2012 .
U.S. troops were in helicopter operations in southern Iraq . Pentagon received report that troops came under mortar fire after crash . Pentagon say no injuries reached yet in crash initially blamed on mechanical failure . Tuesday 's toll worst single - day for U.S. troops since January 2012 .