WASHINGTON – Defense Secretary Jim Mattis acknowledged that the Pentagon was still seeking answers behind the Oct. 4 deaths of four troops in Niger, as calls grew on Capitol Hill for an expanded investigation into the attacks.

“We do not have all the accurate information yet,” Mattis said during a meeting at the Pentagon Thursday with Israeli defense leaders.

Sgt. La David Johnson, Staff Sgt. Bryan Black, Staff Sgt. Jeremiah Johnson and Staff Sgt. Dustin Wright died when their counterterrorism training mission with Nigerien forces was ambushed by what the U.S. military believes to be Islamic State-affiliated militants.

Two other U.S. soldiers were injured. U.S. Africa Command is leading the investigation into the attacks.

Questions remain on whether an intelligence failure on the part of the U.S. left the troops vulnerable, and on the circumstances surrounding Sgt. La David Johnson’s death.

Johnson was not initially retrieved when French helicopter gunships, medevac and a contracted aircraft responded within about a half-hour of the firefight to assist the forces and retrieve the dead and wounded. Johnson remained missing for about two days.

“He was separated,” Pentagon spokesman Dana White told reporters in a briefing late Thursday.

But all facets of the Pentagon, from Mattis down, pushed back on the suggestion that Johnson was left behind as medevac took the other injured and fallen troops.

Lt. Gen. Kenneth McKenzie, director of the Joint Chiefs of Staff, said that Johnson being separated did not mean he was ever left behind. “From the moment of contact, no one was left behind,” McKenzie said.

Mckenzie would not say at what point the U.S. or partnered forces realized Johnson was not with them, but said from the moment of contact, “either U.S. our partnered Nigerien forces or French forces were on the ground actively searching for this soldier. The fact of the matter its a battlefield, we‘d just had a significant engagement,” McKenzie said.

“A lot of men and a lot of women searched very hard to find [Johnson],” McKenzie said.

Mattis said Johnson’s body was found by Nigeriens.

“The U.S. military does not leave its troops behind,” Mattis said. “I just ask that you not question the actions of the troops who were caught in the firefight and question whether or not they did everything they could in order to bring everyone out.”

Troop responses, however, were not what was drawing fire. It was the lack of answers from the Pentagon and White House and a general lack of transparency on where U.S. troops are currently operating receiving the most criticism, lawmakers said.

Senate Armed Services Committee Chair John McCain, R-Ariz., turned up the heat, telling reporters his quest for information from the administration, “might require a subpoena.”

When asked whether the communication problem stems from the Pentagon or the White House, replied, “I think it’s all of the above.”

“I don’t think they have their act together in the Pentagon, much less the White House,” he said. “I know what the problem is, but I don’t know the answer.”

White said that Congress was notified quickly after the incident, something McCain appeared to dispute.

“We did not know about Niger until it came out in the paper,” McCain said. “We need to have a process of communication, which I’ve had with other administrations.”

On Wednesday night, McCain spoke with National Security Advisor H.R. McMaster, urging him to conduct briefings on Capitol Hill, according to Sen. Lindsey Graham, R-S.C., and a close political ally of McCain’s.

“Sen. McCain let the Pentagon and Gen. McMaster know pretty strongly last night that this has to stop,” Graham said. “You’ve got to come over and at least brief the chairman and the ranking member of the relevant committees: Here’s where we’re headed, here’s where we have soldiers deployed.

“You won’t find two stronger defenders of aggressive action in the war on terror than Sen. Graham, but Sen. McCain is right to say this current system is not working,” Graham added.

Graham said he was surprised to learn America had any troops in Niger.

“It’s not that I object, it’s just insist on being informed,” Graham said.

Asked if a subpoena would be necessary to force transparency, the Senate Armed Services Committee’s top Democrat, Rhode Island Sen. Jack Reed, told reporters, “I hope not.

“I mean, we’re just doing our job, which is to look carefully at what took place. We have to authorize these operations and to support them, we have to know what’s going on,” Reed said.

“We haven’t gotten a complete picture of what happened; we need that. In addition, we need to know where there are other train-and-assist missions throughout the area. With Niger, it’s a question of why that went so badly, what can we do, intelligence failures, aviation support.”

Mattis said he would not disclose whether the Pentagon is considering a more aggressive force posture in Africa to better protect trainers and advisers on the ground.

Reed said a more in-depth look at the mission there was required.

“What these troops were doing was train and assist, they thought they were in a benign environment; turned out it was very, very hostile,” Reed said. “This has been a long-term special forces mission. The question of whether we have a strategy has to be answered.”