By Sarah N. Lynch and Mark Hosenball

WASHINGTON (Reuters) – The fatal shooting of three Americans by a Saudi Air Force officer at a Florida naval base last month was “an act of terrorism,” U.S. Attorney General William Barr said on Monday, adding that 21 Saudi military trainees will be pulled out of the United States following an investigation into the incident.

The attack brought fresh complications to U.S.-Saudi relations at a time of heightened tensions between the United States and Iran, Saudi Arabia’s regional rival. Saudi Air Force Second Lieutenant Mohammed Saeed Alshamrani, 21, was fatally shot by a deputy sheriff during the Dec. 6 incident at the facility in Pensacola, Florida.

During a news conference, Barr – the top U.S. 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.

(Reporting By Mark Hosenball; Editing by Andy Sullivan and Howard Goller)