U.S. forces were in helicopter crash in southern Afghanistan . Pentagon received report that troops came under mortar fire after crash . The incident occurred on the outskirts of Kandahar city. Tuesday 's toll worst single-day for U.S. troops since August 2012 .
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 .