Sen. John McCain said Thursday he is holding up President Trump's pick for Army secretary and other nominees partly to get answers about recent soldier deaths in Niger.

McCain, R-Ariz., added the Oct. 4 ambush by ISIS-aligned fighters in West Africa that killed four soldiers to the list of military briefings his Armed Services Committee wants from the Trump administration in exchange for moving Defense Department nominees. The DoD has launched an investigation of the incident in Africa and plans to release the findings to Congress, Lt. Gen. H.R. McMaster, Trump's national security adviser, said Thursday.

The Armed Services chairman has also been pressing Defense Secretary Jim Mattis and the Pentagon to turn over information on a variety of other topics, including Afghanistan and Iraq.

In the meantime, the nomination of Mark Esper, a top Raytheon lobbyist, has languished on the committee's desk since July 25 and McCain said there was no timeline for an Armed Services Committee hearing. Esper was tapped to be the top civilian leader for the largest branch of the military.

"It depends on whether we get the information that we have requested," he said. "I am going to make sure we get the information we need to complete our responsibilities under the Constitution of the United States."

Armed Services has become a bottleneck for the Trump administration as it tries to fill the 39 out of 57 Senate-confirmed leadership posts at the Pentagon that remain empty. There are 19 nominees at the committee as McCain has become increasingly frustrated by what he sees as a lack of transparency from military leaders.

The four soldiers were killed while on an advise and assist mission with Nigerien troops. About 50 fighters waged a surprise attack on the group and the body of one U.S. soldier, Sgt. La David Johnson, was missing for two days.

"If Sen. McCain says we need to do a better job on communicating with him from the departments, from the [National Security Council], we are going to do it. This is a problem we can solve," McMaster said during a security summit.

McMaster said the ongoing investigation will provide answers to lawmakers, the American public and the families of the soldiers who were killed.

"There is a period of time where there is always ambiguity back here in Washington as to what is going on halfway around the world," he said. "On the mission there, the Defense Department will describe what the mission and the circumstances were of that action and of the deaths of those soldiers and all that will come out."