The main gate at Naval Air Station Pensacola is seen on Navy Boulevard in Pensacola, Florida, March 16, 2016. Photo: US Navy / Patrick Nichols / Handout via Reuters.

Twenty-one Saudi military cadets undergoing US training will be ousted following an investigation into the fatal shooting of three Americans by a Saudi officer at a Florida naval base that US Attorney General William Barr on Monday branded an act of terrorism.

The Dec. 6 attack brought fresh complications to US-Saudi relations at a time of heightened tensions between the United States and Iran, Saudi Arabia’s regional rival.

A deputy sheriff shot dead the gunman, Saudi Air Force Second Lieutenant Mohammed Saeed Alshamrani, in the Pensacola, Florida, incident.

Barr said 21 Saudi cadets were “disenrolled from their training curriculum” in the US military and would leave the United States later on Monday after an investigation showed they either had child pornography or social media accounts containing Islamic extremist or anti-American content.

During a news conference, Barr — the top US law enforcement official — said there was no evidence of assistance by other Saudi trainees or that any of them had knowledge in advance of the attack.

“This was an act of terrorism,” Barr said. “The evidence showed that the shooter was motivated by jihadist ideology. During the course of the investigation, we learned that the shooter posted a message on Sept. 11 of this year stating, ‘The countdown has begun.'”

Barr added that Alshamrani also visited the New York City memorial to the victims of the Sept. 11, 2001, attacks on the United States and posted anti-American, anti-Israeli and jihadi messages on social media, including two hours before the attack.

Saudi Arabia provided “complete and total support” to the American investigation of the incident, Barr said.

Barr said one Saudi individual had “a significant number of such images,” while 14 others “had one or two images, in most cases posted in a chat room by someone else or received over social media.

The “derogatory information” fell short of the standard for triggering US criminal charges, Barr said.

“However, the Kingdom of Saudi Arabia determined that this material demonstrated conduct unbecoming an officer in the Royal Saudi Air Force and in the Royal Navy,” Barr said, adding that “the 21 cadets have been disenrolled from their training curriculum in the US military and will be returning to Saudi Arabia later today.”