Sen. John McCain (R-Ariz.) returns to his office after votes at the Capitol in Washington on Oct. 19, 2017. (J. Scott Applewhite/AP)

The Senate’s top Republican on military matters threatened Thursday to subpoena the Trump administration if officials are not more forthcoming about the Niger attack that left four American service members dead — just one of the steps lawmakers are taking to insist that Congress be read in on military operations before tragedies occur.

Senate Armed Services Committee Chairman John McCain (R-Ariz.) is pushing the Trump administration to brief key members of Congress about the existence of ongoing operations — something he said the Obama administration was far better about doing than the Trump team.

“There’s a mind-set over there that they’re a unicameral government,” McCain said on Thursday, accusing the Trump administration of intentionally trying to keep Congress in the dark about the military’s foreign engagements and noting that “it was easier under Obama.”

“We are coequal branches of government; we should be informed at all times,” he added. “We’re just not getting the information in the timely fashion that we need.”

McCain communicated those frustrations to national security adviser H.R. McMaster during a Wednesday afternoon meeting with Armed Services Committee members. While McMaster seemed sympathetic to his demands that Congress be better informed, McCain was unconvinced that that would lead to any policy changes, noting that “talk is cheap.”

Four U.S. soldiers were killed in Niger Oct. 4, after an attack near Niger’s border with Mali. Here's what we know. (Victoria Walker/The Washington Post)

Tensions between lawmakers and the Trump administration about how extensively key lawmakers such as McCain are briefed on active operations have flared in recent weeks after four U.S. Special Forces soldiers died in an ambush in Niger. Staff Sgt. Bryan Black, Staff Sgt. Jeremiah W. Johnson, Staff Sgt. Dustin Wright, and Sgt. La David Johnson had been in Niger as part of a counterterrorism mission to provide advice and training to local forces and were not expected to come into contact with enemy fighters.

Lawmakers on Capitol Hill have yet to be briefed about the particulars of the attack — a delay that led McCain to accuse the administration this week of not being upfront with Congress about the particulars of the ambush. On Thursday, McCain added that he was prepared to use “everything, everything, everything” at his disposal to get complete information about the attack, even if “it may require a subpoena.”

McMaster, he added, promised on Wednesday to brief lawmakers soon.

But the lawmakers’ frustrations run deeper than simply being underinformed about one attack. McCain and Sen. Lindsey O. Graham (R-S.C.) said Thursday that before the news of the ambush, they had almost no knowledge about what U.S. Special Forces were doing in Niger.

“Very little,” McCain said, when asked whether he knew anything about the military’s mission there. He surmised that there were likely to be other troops deployed in global operations that the committee had not been made aware of, “but I don’t know who they are.”

Graham, who also met with McMaster on Wednesday, said that his knowledge of U. S. forces’ work in Niger was “not in any great detail, just in general.”

“I’m all for going after terrorists,” Graham added, “but I want to know before I read about it in the paper where our people are and what they’re doing.”

White House Chief of Staff John F. Kelly addressed the confusion surrounding the events in Niger on Thursday, telling reporters that “there’s an investigation ongoing. An investigation doesn’t mean anything was wrong. An investigation doesn’t mean people’s heads are going to roll. The fact is they need to find out what happened and why it happened.”

But he did not promise to expedite getting information out to the public, adding: “I’ve read the same stories you have. I actually know a lot more than I’m letting on, but I’m not going to tell you.”

The FBI is assisting in the investigation, a U.S. official said Thursday. The official declined to describe the particular assistance, first reported by the Wall Street Journal, but characterized it as routine and noted that the bureau has personnel in Africa. The FBI has assisted in past military investigations, including the killing of U.S. soldiers in Jordan.

McCain said Thursday that he was ready to be briefed by administration officials in a classified or public setting, depending on the issue. But, he stressed, the Pentagon must be more forthcoming.

“It’s not wanting [more information],” he said. “It’s a requirement that the Senate Armed Services Committee have oversight of the military.”

The dispute over disclosure of operations is just one of the latest arenas in which McCain has tangled with the Trump administration over its defense policy. He has accused the Trump team of not living up to its promises to better fund the military in its budget request — a subject that is gripping Congress as members hash out the particulars of a budget and an annual defense authorization bill. McCain has excoriated the administration as being unprepared for the aftermath of the Islamic State in the Middle East and as too soft on Russia after Moscow’s attempts to challenge Washington on the world stage and meddle in various government systems and the 2016 U.S. election.

McCain has also periodically chastised the administration for complicating his congressional oversight responsibility — at first, because the Pentagon was sluggish about filling vacant positions, and more recently, because the Pentagon has not furnished certain government officials for committee interviews.

On Thursday, McCain noted with irritation that the Trump administration had refused to let Rob Joyce, the White House’s top cybersecurity official, testify at a hearing devoted to examining the country’s defenses against cyberattacks.

“Unfortunately, but not surprisingly, the White House declined to have its cyber coordinator testify,” McCain complained. “To me, the empty chair before us represents a fundamental misalignment between authority and accountability in our government today when it comes to cyber.”

Karen DeYoung, Matt Zapotosky and Devlin Barrett contributed to this report.