The shooting carried out by a Saudi aviation student at Naval Air Station Pensacola in December was an “act of terrorism,” Attorney General William Barr said Monday as authorities released their findings on an investigation into the attack.

Barr said the gunman, identified as Saudi Arabian Air Force 2nd Lt. Mohammed Saeed Alshamrani, was "motivated by jihadist ideology."

He added that Alshamrani, during last year's anniversary of the September 11, 2001 terrorist attacks, posted a message online that "the countdown has begun" and later traveled to New York City to visit the 9/11 Memorial on Thanksgiving weekend.

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More than a dozen Saudi nationals who live in America and train with the U.S. military -- and have ties to extremist groups -- are set to be sent back home Monday, a law enforcement source tells Fox News.

The announcement comes as investigators are wrapping up their probe of December’s mass shooting at Naval Air Station Pensacola, which left three U.S. sailors dead and was carried out by a 21-year-old Saudi aviation student training there. Attorney General William Barr is scheduled to speak about the matter later this afternoon.

The law enforcement source says “more than a dozen” Saudi nationals who have ties to extremist groups will be expelled from the U.S. Those individuals, who train with the U.S. military, are not linked to the Pensacola shooting though, the source added.

NAS Pensacola is home to the Naval Education and Training Security Assistance Field Activity's International Training Center, which the Navy says was “established in 1988 to meet the aviation-specific training needs of international officers and enlisted students from allied nations.”

"Immersing international students in our U.S. Navy training and culture helps build partnership capacity for both the present and for the years ahead," Cmdr. Bill Gibson, the center’s officer in charge, said in 2017. "These relationships are truly a win-win for everyone involved."

The majority of the hundreds of foreign aviation students who have participated in the program are from Saudi Arabia, the Navy says. The Naval training program has about 1,500 pilots in total.

Saudis have received training at the Pensacola site since the 1970s, with as many as 20 students from the Middle Eastern country in any given class, sources told Fox News. Many of the students are often from the Royal Family, putting pressure on officials to pass pilots through the training program in an attempt to preserve diplomacy with the U.S. ally.

The shooting at NAS Pensacola on Dec. 6 also prompted a group of U.S. Navy instructor pilots to ask top military brass for permission to arm themselves.