A longer-than-expected meeting with local tribal leaders in Niger may have given militants critical extra minutes to prep the ambush attack that left four American troops dead earlier this month, two U.S. officials told Fox News on Friday.

Defense Secretary James Mattis was set to meet with Sen. John McCain, R-Ariz., on Capitol Hill on Friday afternoon to discuss the incident, the officials said. Mattis' visit comes one day after McCain threatened to subpoena top Pentagon officials over their response to the attack.

Asked by Fox News on Thursday if the administration has been forthcoming about the attack, McCain replied, “of course not” and added, “it may require a subpoena.”

It was not immediately clear why American forces and Nigeriens arranged the Oct. 4 meeting, but the founder of a non-profit group told Fox News the area of the attack was volatile.

“All [Non-government organizations] pulled out of the area [where the attack took place] a few months ago,” impl. project's Justin Richmond said.

“There’s literally no one there with a sustained presence,” which opens the door for terrorist groups to move in the area, he added. “The French and Malian forces pushed [Al Qaeda-linked] Ansur Dine and Macina Liberatino Front out of Mali and into western Niger.”

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Pentagon officials still suspect an ISIS-linked group is behind the attack, but are not ruling out any Al Qaeda affiliated organizations.

A dozen U.S. Army soldiers, mostly Green Berets, along with 30 Nigeriens, had traveled 125 miles north from their base at Niger’s capital, Niamey, in unarmored trucks on a routine mission and to meet with local village elders in Tonga Tonga, near the border with Mali.

After the meeting with the village elders ended, the U.S.-led patrol was ambushed by roughly 50 militants.

French aircraft were overhead within 30 minutes, however, they did not fire because they couldn't positively identify who was who on the ground.

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A senior defense official told Fox News the U.S. troops were fired on once they were already in their vehicles. The vehicles then scrambled to “get off the X” -- escaping the ambush site using evasive driving maneuvers -- and a gunfight ensued.

Mattis said Thursday the attack is still under investigation, and the Pentagon has dispatched a general officer to Niger to probe what happened.

Fox News’ Lucas Tomlinson contributed to this report.