The 21, who the investigation found possessed jihadist material and child pornography, have been expelled from their air force training at a Florida military base, said Attorney General Bill Barr.

"The Kingdom of Saudi Arabia determined that this material demonstrated conduct unbecoming an officer in the Saudi Royal Air Force and Royal Navy and the 21 cadets have been dis-enrolled from their training curriculum in the US military and will be returning to Saudi Arabia later today," Barr said.

Barr called the December 6 shooting by Royal Saudi Air Force 2nd Lieutenant Mohammed Saeed Alshamrani an "act of terrorism."

"The evidence shows that the shooter was motivated by jihadist ideology," he said.

Alshamrani killed three US sailors and wounded eight other people in a classroom building at the Pensacola Naval Air Station before being shot dead by police.

Barr said Alshamrani had posted a message on social media on September 11, 2019 saying "the countdown has begun," in addition to several other anti-US, anti-Israel, and jihadist messages.

Barr also stressed that the Saudi government and the other Saudis in the training program cooperated fully with the investigation into the shooting, which threatened a decades-old military training program crucial to the US-Saudi relationship.