Updated at 1:25 p.m. ET

The Pentagon says three U.S. service members were shot and killed Friday at a Jordanian military base, reportedly fired on as their vehicles entered the facility.

A U.S. official told NPR's Tom Bowman that it was Jordanian soldiers who opened fire. The reason wasn't immediately clear, the official says, but there were no apparent signs of hostile intent.

Initial reports suggested that one service member died and two were injured after coming under small-arms fire, the official said. But the two others died after they were transported to a hospital in Jordan's capital, Amman.

The incident happened outside the King Faisal Airbase in southern Jordan, according to Jordan's state news agency. It added that a Jordanian officer was also injured and that Jordan has launched an investigation.

A spokesperson for the U.S. Embassy in Amman tells NPR's Jane Arraf that Jordanian authorities "have offered their full support."

Jordan, a relatively stable country bordering Syria and Iraq, still "faces homegrown extremism, with hundreds of Jordanians fighting alongside IS militants in Iraq and Syria and several thousand more supporting the extremist group in the kingdom," The Associated Press reported.

Jordan is part of the U.S.-led coalition fighting ISIS in Iraq and Syria, Tom has reported, and "has hosted training for Syrian rebels on its own soil."

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