Attorney General William Barr said last month's shooting at Naval Air Station in Pensacola was "an act of terrorism."
Barr, during a news conference Monday afternoon, said investigators have concluded that there was jihadist influence.
The shooter posted a message on social media in September that stated "the countdown has begun."
Several other messages were posted on social media, including a post two hours before the attack, Barr said.
Three people were killed and several others were injured in the Dec. 6 shooting.
The gunman, a member of the Saudi Arabian military who was training at the base, was also killed. He reportedly visited the 9-11 memorial in New York City in the days before he killed three American sailors.
The Saudis are not accused of aiding the 21-year-old Saudi Air Force second lieutenant, but more than a dozen Saudi servicemen training at US military installations will be expelled from the United States following the attack.
Naval Air Station Pensacola employs more than 16,000 military and 7,400 civilian personnel, according to the base's website.