Sen. 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.) said Thursday it might require a subpoena to get more information about a deadly ambush in Niger.

"It may require a subpoena," McCain said when he was asked what might need to be done to get more information about the attack that left four U.S. soldiers dead, according to CNN.

U.S. Africa Command has launched a formal investigation into the Oct. 4 incident in Niger that left four Army Green Berets dead and two injured, according to reports Thursday.

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The command sent a team to Niger to conduct a "review of the facts," defense officials told NBC News.

Experts are reportedly trying to establish an hour-by-hour timeline of what happened, and the investigation will include all U.S. military branches and agencies involved in the deadly mission.

McCain said it's not necessary to wait for the Defense Department to finish its investigation.

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

McCain told The Hill on Thursday that the committee was going to meet to decide how to move forward, noting that its members didn't find out about what happened in Niger "until it came out in the paper."

"I hope not. We're just doing our job, which is to look carefully at what took place. We have to authorize these operations and support them, and we have to know what's going on," Reed told The Hill on Thursday.

On Tuesday, McCain said he didn't think the Trump administration had provided enough information on the attack in Niger.

Asked by The Hill whether the administration has been forthcoming on information on Niger, McCain said "no."

“I had a better working relationship, as far as information back and forth, with Ash Carter than I do with an old friend of 20 years,” McCain added, referring to former President Obama's last Defense secretary.

President Trump did not personally comment on the attack until Monday, when he said he would call the families of the fallen soldiers "at some point" and spurred controversy over his claim that Obama didn't call the families of slain soldiers.