Investigators found a history of the shooter posting radical content online.

Federal investigators have determined that the shooting at a Florida air base that killed three people was likely a “terror” attack inspired by radical Islamic ideology, three sources briefed on the ongoing investigation told ABC News.

On Dec. 6, Mohammed Alshamrani, 21, a second lieutenant in the Saudi Air Force, allegedly opened fire inside a classroom at Air Base Pensacola, killing three U.S. Navy personnel and injuring another eight people. Alshamrani was shot and killed by Escambia County Sheriff’s deputies at the scene.

Alshamrani was in the United States for flight training and purchased the Glock 9 mm pistol he used in the attack about four months ago by taking advantage of a federal gun exception that allows foreign nationals to legally purchase weapons for hunting, authorities said.

Alshamrani allegedly followed radical Islamist content online, including sermons by the American-born cleric Anwar al Awlaki, who was killed in 2011 in a CIA-led drone strike.

Alshamrani traveled to New York city shortly before the attack, authorities said, but the travel was likely related to his training and not an effort by him to seek out potential targets.

The U.S. Department of Defense has since suspended operational training for all Saudi students in the wake of the shooting.

The FBI has not officially designated the shooting as a terror attack, saying only that it is being investigated with the “presumption” that it was an act of terror.

Federal investigators are also probing a report that Alshamrani watched mass-shooting videos in the presence of some friends in the days leading up to the attack, two sources briefed on the probe told ABC News.

ABC News’ Bill Hutchinson and Aaron Katersky contributed to this report.