Investigators found evidence of the communications on the cellphone of Mohammed Alshamrani, 21, a Royal Saudi Air Force flight student who killed three US sailors at the Naval Air Station Pensacola in Florida on December 6, according to CNN and the New York Times.

US Attorney General Bill Barr and FBI Director Christopher Wray were expected to announce the discovery at a press conference later Monday in Washington.

Alshamrani, one of dozens of Saudis studying at the base at the time, was encouraged by the Qaeda operative to carry out the attack, according to the reports.

Alshamrani had posted on his now-suspended Twitter account condemnations of the United States as a "nation of evil" as well as a quote from Al-Qaeda's deceased founder Osama bin Laden.

Weeks after the attack, the Yemen-based cell Al-Qaeda in the Arabian Peninsula (AQAP) claimed credit for the attack.

In February, the United States said it had recently killed AQAP's leader Qassim al-Rimi in a secret operation.