The FBI has joined the investigation into how four American soldiers were killed by Islamic militants in Niger two weeks ago, as the Pentagon and White House face growing questions about the attack, The Wall Street Journal reported Thursday.

FBI officials told the Journal that the bureau's involvement is not unusual, and that it has the authority to take charge of the probe, but has not yet done so.

The development comes as lawmakers on Capitol Hill demand answers as to how militants were able to ambush the U.S. soldiers, and as Trump faces scrutiny for his handling of the aftermath of their death.

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The four Americans were killed and two more were injured when their group of about a dozen U.S. soldiers and 40 Nigerien troops were ambushed during a joint patrol.

The Pentagon has laid blame on self-radicalized, Islamic State in Iraq and Syria-affiliated militants for the attack.

Some lawmakers, including chairman of the Senate Armed Services Committee 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.), have called on the administration to provide more details on the attack.

“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 told reporters this week, referring to the Defense secretary under former President Obama.

“We’ve been waiting for weeks and weeks,” he said. “We will not sit by without having a complete understanding of what’s going on.”

Pentagon officials insisted on Thursday that they were being forthcoming with lawmakers about the attack.

"The loss of our troops is under investigation," Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE told reporters. "We investigate anytime we have our troops killed, whether it be in a training accident or combat."

"These terrorists are conducting war on innocent people of all religions, they are conducting war on innocent people who have no way to defend themselves," he continued.

"In this specific case, contact was considered unlikely, but there's a reason we have U.S. Army soldiers there and not the Peace Corps, because we carry guns," he said.