The Saudi gunman who killed three people at the Pensacola naval base had apparently gone on Twitter shortly before the shooting to blast U.S. support of Israel and accuse America of being anti-Muslim, a U.S. official said Sunday as the FBI confirmed it is operating on the assumption the attack was an act of terrorism.

Investigators are also trying to establish whether the killer, 2nd Lt. Mohammed Alshamrani, 21, of the Royal Saudi Air Force, acted alone or was part of a larger plot…

Authorities believe the gunman made social media posts criticizing the U.S. under a user handle similar to his name, but federal law enforcement officials are investigating whether he authored the words or just posted them, said the official, who was not authorized to discuss the matter publicly and spoke to The Associated Press on condition of anonymity.

Also, investigators believe the gunman visited New York City, including Rockefeller Center, days before the shooting and are working to determine the purpose of the trip, the official said.