Washington — Members of Congress demanded answers on Thursday two weeks after an ambush in the African nation of Niger killed four U.S. soldiers, with one top lawmaker even threatening subpoenas. The White House defended the slow pace of information, saying an investigation would eventually offer clarity about a tragedy that has morphed into a political dispute in the United States.

Among the unresolved inquiries: Why were the Americans apparently caught by surprise? Why did it take two additional days to recover one of the four bodies after the shooting stopped? Was the Islamic State responsible?

The confusion over what happened in a remote corner of Niger, where few Americans travel, has increasingly dogged President Donald Trump, who was silent about the deaths for more than a week.

The deadly ambush in Niger occurred as Islamic militants on motorcycles, toting rocket-propelled grenades and heavy machine guns, seized on a U.S. convoy and shattered the windows of their unarmored trucks. In addition to those killed, two Americans were wounded. No extremist group has claimed responsibility.

The attack is under official military investigation, as is normal for a deadly incident.

What is abnormal, according to Sen. John McCain, the Republican chairman of the Senate Armed Services Committee, is the Trump administration’s slow response to requests for information. He said Thursday it may take a subpoena to shake loose more information.

“They are not forthcoming with that information,” McCain told reporters.

Sen. Bob Corker, the Tennessee Republican who chairs the Senate Foreign Relations Committee, said members of Congress have been provided with some information about the attack, “but not what we should.”

At the Pentagon, Defense Secretary Jim Mattis pushed back, saying it naturally takes time to verify information about a combat engagement. He promised to provide accurate information as soon as it’s available, but offered no timetable.

“The loss of our troops is under investigation,” he said. “We in the Department of Defense like to know what we’re talking about before we talk.”

Mattis did not offer details about the circumstances under which the Americans were traveling but said contact with hostile forces had been “considered unlikely.”

That would explain why the Americans, who were traveling in unarmored vehicles with Nigerien counterparts, lacked access to medical support and had no immediate air cover, although Mattis said French aircraft were called to the scene quickly. He said contract aircraft flew out the bodies of three Americans shortly after the firefight. Local Nigeriens found Johnson’s body and returned it Oct. 6.

It’s not clear why Johnson was not found with the three others Oct. 4. Dana W. White, a spokeswoman for Mattis, said Johnson had become “separated.” Speaking at a news conference with her, Lt. Gen. Kenneth McKenzie, director of the Joint Staff, said he knew more about what had happened to Johnson but was not willing to share it.