US authorities are looking into recent travel to the Middle East by Mohammad Youssef Abdulazeez, who shot and killed five US Marines on Thursday in Chattanooga, Tenn., before he was shot dead by police.

“We do know that he has traveled outside of the United States,” said Ed Reinhold, the FBI agent leading the probe into the shooting.

Authorities fear that Abdulazeez was radicalized in recent years and are investigating his visit to Jordan last year. They have also not ruled out a possible trip he made to Yemen.

Abdulazeez was born in Kuwait to a Palestinian father and was a naturalized American citizen. The FBI and police have said it is “premature” to declare his motive for the shooting, but they have already called the attack “domestic terrorism.”

“It is a heartbreaking circumstance for these individuals who have served our country with great valor to be killed in this fashion,” President Barack Obama said.

The American Jewish Committee (AJC) called for greater vigilance in light of the growing threat of Islamic terrorism on US soil.

“Chattanooga tragically is not the first time terrorists inspired by a twisted, hateful, violent Islamist ideology have targeted and killed American military on US soil,” said AJC Executive Director David Harris. “Constant vigilance, of course, remains absolutely essential in seeking to prevent such heinous attacks and apprehending anyone who threatens Americans before they strike.”