Barr calls on Apple to unlock Pensacola gunman's iPhones

"This was an act of terrorism," Barr said of the Dec. 6 incident at the facility in Pensacola, Florida, in which a deputy sheriff shot dead the gunman, Saudi Air Force Second Lieutenant Mohammed Saeed Alshamrani.

During a news conference, Barr - the top U.S. law enforcement official - said there was no evidence of assistance by other Saudi trainees or that any of them had knowledge in advance of the attack.

But Barr said the Federal Bureau of Investigation had not received any "substantive assistance" from Apple in its efforts to find potential evidence on two iPhones belonging to Alshamrani.

"This situation perfectly illustrates why it is critical that the public be able to get access to digital evidence," Barr said, adding that "it's becoming a grave problem."