"It may require a subpoena," McCain told reporters on Capitol Hill when asked what steps he would be willing to take to get more information.
The Defense Department is conducting an initial review of the deadly incident, searching for precise answers as to how 50 ISIS-affiliated fighters were able to ambush a group of soldiers two weeks ago.
McCain, who is the chairman of the Senate armed services committee, said he might not wait for the department to finish its own investigation before seeking those details.
"That's not how the system works. We're coequal branches of government," McCain told CNN's Suzanne Malveaux. "We should be informed at all times."
The longtime senator forcefully argued that the Trump administration has not been forthcoming and needs to do a better job of keeping the committee up to speed.
"We did not know about Niger until it came out in the paper. We need to have a process of communications, which I've had with other administrations, of exchanging information and knowledge," he told reporters.
However, he said he had a "good conversation" with national security adviser H.R. McMaster Wednesday night and they're currently in the process of scheduling a briefing on the matter.
Asked Thursday how he felt about McCain's criticism -- that it was easier to get information under previous administrations -- McMaster said, "It hurt my feelings."
"I love and respect Sen. McCain," he said. "if Sen. McCain says we need to do a better job communicating with him...we're going to do it."
McCain also signaled that he might hold up defense nominations over the issue. He said he hadn't heard back from the administration after he and Democratic Sen. Ben Cardin sent a letter last week asking why recently approved sanctions against Russia haven't been implemented yet.
"Why do you think they go past the deadline on all of these things?" McCain asked reporters. "Because they think they can get away with it. That's why we say no nominations and we get the information we need."
Asked if he was holding up nominations until he gets answers just on Russia sanctions, McCain said, "On everything."
The Arizona Republican has not gone into detail about what kind of information he was looking for on Niger, saying only that he was interested in "all the specifics."
"That's why we're called the Senate armed services committee. It's because we have oversight of our military," he said. "So we deserve to have all the information."
Sen. Jack Reed, the ranking member of the Senate armed services committee, did not express strong agreement with McCain on the idea of a potential subpoena, though he also wanted more information.
"I hope not," he said. "We're just doing our job, which is to look carefully at what took place. We have to authorize these operations and support them. We have to know what's going on."
The comprehensive investigation of the timeline has been ordered
by US Africa Command and includes all the military branches and elements of US intelligence agencies that were involved in the mission. Team members who were on the ground are being interviewed about what happened as well as preparations for the mission.
Sen. Mike Rounds, a Republican on the committee, said he felt it was "appropriate" to let the Defense Department finish its investigation before the committee took its next steps.
"We'll allow General Mattis to do his investigation," he said. "At that point, whether it be in a classified session or an unclassified hearing, I would suspect that we'll have a report delivered."
White House spokeswoman Sarah Sanders was asked in the press briefing Wednesday whether President Donald Trump
was satisfied with the information he has received about the mission and ambush.
"I believe they're still looking into the details of that," Sanders replied. "But I don't think that the President can ever be satisfied when there's loss of life from men and women in uniform."