Authorities investigating a deadly attack at a U.S. naval base in Florida are reportedly focused on finding several unaccounted for Saudi nationals linked to the shooting, as additional details have emerged about the shooter’s movements in the weeks leading up to the rampage.

The FBI’s Jacksonville office identified the shooter in a statement Saturday night as Mohammed Alshamrani, 21, and released a photo of him. Investigators said he was a 2nd Lt. in the Royal Saudi Air Force and was a student naval flight officer of Naval Aviation Schools Command when he opened fire Friday at Naval Air Station Pensacola in Florida before being shot dead.

In the days since the attack, a U.S. official who spoke on condition of anonymity after being briefed by federal authorities said a total of 10 Saudi students were being held on the base as of Saturday while several others were unaccounted for.

While officials have not publicly disclosed how many missing servicemen are out there, U.S. Northern Command (Northcom) has called for increased random security checks at all sites as authorities investigating the attack are still working to determine whether the shooting was an act of terrorism.

“There’s no question what it is, it’s terrorism.” Florida Sen. Rick Scott (R) said. “It’s radical Islam.”

Lt. Cmdr. Michael Hatfield said after the shootings last week at Joint Base Pearl Harbor-Hickam in Hawaii and in Florida, Northcom has directed all Defense Department installations, facilities and units within the U.S. to immediately assess force protection measures and “implement increased random security measures appropriate for their facilities.”

“The advisory also told leaders to remind their workforce to remain alert and if they see something, to say something by immediately reporting to appropriate authorities any suspicious activity they may observe,” Hatfield continued.

On Saturday, a U.S. official said Alshamrani hosted a dinner party earlier in the week where he and three others watched videos of mass shootings. The official who spoke Saturday said one of the three students who attended the dinner party hosted by the attacker recorded video outside the classroom building while the shooting was taking place on Friday. Two other Saudi students watched from a car, the official said.

The Navy on Saturday identified the three victims of the NAS Pensacola shooting as Ensign Joshua Kaleb Watson, 23, of Coffee, Ala.; Airman Mohammed Sameh Haitham, 19, of St. Petersburg, Fla.; and Airman Apprentice Cameron Scott Walters, 21, of Richmond Hill, Ga.

“The sailors that lost their lives in the line of duty and showed exceptional heroism and bravery in the face of evil,” Capt. Tim Kinsella, the commanding officer of Naval Air Station Pensacola, said in a statement. “When confronted, they didn’t run from danger; they ran towards it and saved lives.”

Kinsella said Naval Air Station Pensacola, one of the Navy’s most historic and storied bases, would remain closed until further notice. The base sprawls along the waterfront southwest of the city’s downtown and dominates the economy of the surrounding area.

Part of the base resembles a college campus, with buildings where, in addition to foreign students, 60,000 members of the U.S. Navy, Marines, Air Force and Coast Guard train each year in multiple fields of aviation.

The FBI’s Jacksonville office has said it’s not aware of any credible threat toward the Pensacola community at this time, but anyone with information regarding Alshamrani and his activities before the shooting is encouraged to call 1-800-CALL-FBI.