The growing list of unanswered questions and inability to construct a precise account of the Oct. 4 incident have exacerbated a public relations nightmare for the White House, which is embroiled in controversy over President Trump's belated and seemingly clumsy response this week to console grieving military families.

"We need to find out what happened and why," White House Chief of Staff John F. Kelly, whose son was killed in Afghanistan in 2010, told reporters at the White House on Thursday.

At the Pentagon, Mattis suggested to reporters that he would say little pending results of the investigation. "We at the Department of Defense like to know what we're talking about before we talk," he said. "And so we don't have all the accurate information yet. We will release it as rapidly as we get it."

The attack, apparently carried out by militants affiliated with Islamic State, was the deadliest since Trump took office, yet the U.S. military's Africa Command still does not have a clear "story board" of facts that commanders usually gather swiftly after deadly incidents.