Pentagon officials insisted on Thursday that they were being open with Congress while briefing lawmakers on the attack in Niger that left four U.S. special forces dead and two others injured.

Pentagon chief spokeswoman Dana White said the Defense Department had a military general brief members of the House and Senate Armed Services committees in a closed session on the Oct. 4 ambush. She did not say who spoke to the committees.

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The briefing came the same day Senate Armed Services Committee Chairman John McCain John Sidney McCainMomentum growing among Republicans for Supreme Court vote before Election Day McConnell urges GOP senators to 'keep your powder dry' on Supreme Court vacancy McSally says current Senate should vote on Trump nominee MORE (R-Ariz.) told reporters it might require a subpoena to get more information about the Niger attack.

McCain earlier in the week had said he didn't think the Trump administration had provided enough information on the incident.

And Senate Foreign Relations Committee Chairman Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.) said lawmakers have not been given “what we should” as far as information.

“We have kept them up to date. Of course we will work with Sen. McCain and his staff to ensure they get everything that they need,” White told reporters at the Pentagon. “It is very important to [Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE] and he is personally dedicated to that.”

White added that the Defense Department notified the leadership and staffs of both committees of the attack the afternoon of Oct. 4, the day it occurred. The Pentagon also “updated them both via phone calls and emails throughout the situation.”

“I would say that we have done all we can and will continue to strive to do as much as we can to ensure that Sen. McCain and all the members of the [Senate Armed Services Committee] and the [House Armed Services Committee] have exactly what they need when they need it,” she said.

The U.S. military has launched a formal investigation into the ambush, during which a dozen U.S. soldiers from the 3rd Special Forces Group and 30 to 40 Nigerien troops were attacked during a routine train-and-assist mission. The group was surprised along the border between Niger and Mali when roughly 50 Islamic State in Iraq and Syria-affiliated fighters began firing on them.

Mattis confirmed the investigation Thursday at the Pentagon before meeting with his Israeli counterpart. The former Marine Corps general suggested little more will be said on the ambush until the findings come out.

“A full investigation is underway, the sort of investigation we always conduct,” he told reporters. “We’ll certainly update you as we have information, accurate information, not speculation.”

“We at the Department of Defense like to know what we’re talking about before we talk,” he added.

But McCain said it's not necessary to wait for the department to finish its investigation.

"That's not how the system works. We're coequal branches of government," said McCain. "We should be informed at all times."