Congresswoman Frederica Wilson and Senator John McCain demand clarity over ambush that left four dead as Pentagon says it is investigating

Donald Trump’s administration is facing mounting pressure to release more details about the circumstances that led to the ambush of US troops in Niger, leaving four Americans dead and two wounded.

The controversy surrounding the Niger attack escalated this week as the White House engaged in a high-profile feud with Frederica Wilson, a Democratic congresswoman, over Trump’s condolence call to the widow of one of the fallen soldiers.

But questions have also compounded over what exactly transpired in the north-west African country, with critics accusing the administration of failing to provide adequate information on the deadliest combat mission since Trump took office in January.

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Frustration over the lack of answers spilled into public view on Thursday, as the Arizona senator John McCain suggested the issue “may require a subpoena”. Asked by reporters on Capitol Hill if the Trump administration had been forthcoming about the attack, McCain, who chairs the Senate armed services committee, replied: “Of course not.”

In separate remarks on Thursday, the defense secretary, James Mattis, said the Pentagon was conducting an investigation into the attack while noting the US currently had roughly 1,000 troops stationed in the region to provide support and training for a French-led mission aimed at eradicating extremist groups.

While no group has formally claimed responsibility for the deadly attack on 4 October, US officials have said it was probably carried out by an Islamic State affiliate. Initial reports have suggested that a 12-member team of US troops were traveling in unarmored trucks when they were ambushed by as many as 50 militants.

Mattis told reporters US intelligence had deemed it “unlikely” that the troops would encounter hostile forces, while adding: “I would just ask that you not question the actions of the troops who were caught in the firefight and question whether or not they did everything they could to bring everyone out at once.”

Mattis also said French aircraft responded to the situation and evacuated the wounded Americans, along with those killed.

But Sgt La David Johnson, whose widow received the phone call from Trump that has been at the center of its own controversy, was separated from the group and therefore not among those recovered on the day of the attack.

His body was found by local residents and returned on 6 October. It was not yet clear if Johnson was alive at the time French helicopters left the scene, nor had it been explained why it took 48 hours to locate his body.

It was confirmed on Friday that the FBI had joined the investigation into the Niger attack, a standard practice when American citizens are killed overseas.

Mattis emphasized that the US military “does not leave its troops behind”, but the response thus far has been insufficient for critics such as Wilson, who was close to Johnson’s family.

Mattis met privately with McCain at his Capitol Hill office on Friday in what appeared to be an effort to quell some of the senator’s concerns.

“We can do better at communication, we can always do better on communication, and that’s what we’ll do,” Mattis told reporters after the meeting.

Asked if Trump had authorized the US mission in Niger, Mattis would not say.

“I don’t discuss those kinds of things,” he said.

Wilson, who disclosed this week that Trump had told Johnson’s widow the fallen solder “knew what he signed up for” in a condolence call, told CNN on Friday she was seeking further explanation.

“He was abandoned for two days, for 48 hours,” Wilson said of Johnson, while adding she had requested a classified briefing on the incident.

“I want to know why he was separated from the rest of the soldiers,” she said. “Why did it take 48 hours for them to find him? Was he still alive? Was he kidnapped?

“I am distraught and so is the family. There’s so many questions that must be answered.”

Trump has also come under fire for taking nearly two weeks to make any public mention of the attack. Speaking at the White House Rose Garden on Monday, the president addressed Niger for the first time while noting he had written letters to families of the fallen soldiers.

In the subsequent days, however, the focus shifted toward Trump’s false claim that Barack Obama and George W Bush had not reached out to Gold Star families.

The president also fired back at Wilson, suggesting multiple times on Twitter that the Florida Democrat’s account of his call with Johnson’s widow was “fabricated”. Trump continued to make the argument even after Kelly essentially confirmed the contents of the call in remarks on Thursday, while seeking to defend the president’s tone.

Left unaddressed amid the furor was whether Trump authorized the US mission in Niger, a country that prohibits offensive air operations.

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In a sign that Trump’s war authorization powers might be the subject of scrutiny on Capitol Hill, the Senate foreign relations committee on Friday announced plans to hold a public hearing on the use of military force. Mattis and the secretary of state, Rex Tillerson, are expected to testify.

The Trump administration, as the Obama administration did, has been relying on the authorization for use of military force (AUMF) passed by Congress in 2001 to authorize the war in Afghanistan. Senators Jeff Flake and Tim Kaine, both members of the foreign relations committee, have been pushing for a vote to replace the AUMF with a new measure to authorize military force against al-Qaida, the Taliban and Islamic State.

On Friday, Kaine said the attack in Niger reinforced the need for a debate about war powers, stating: “The many questions surrounding the death of American service members in Niger show the urgent need to have a public discussion about the current extent of our military operations around the world.”

He added: “For sixteen years, Congress has remained largely silent on this issue, allowing Administrations to go to war anywhere, anytime. A new AUMF is not only legally necessary, it would also send an important message of resolve to the American public and our troops that we stand behind them in their mission.”