Barr said investigators found that Alshamrani, a Saudi air force member who was training at the base, posted a message on September 11th saying "the countdown has begun." The gunman "posted other anti-American, anti-Israeli and jihadi messages on social media, and did so two hours before his attack," according to Barr. Alshamrani killed three and wounded eight others before responding officers shot him dead.

Barr has argued the government should have backdoor access to encrypted communications in the interest of public safety, and last month described that as one of his department's "highest priorities." However, Apple is said to be standing firm in its support of encryption, which could set it on a path towards another high-profile privacy battle with authorities.

Were the government to be successful in its "dangerous and unconstitutional" demand, it would "weaken the security of millions of iPhones," according to the American Civil Liberties Union.

"Strong encryption enables religious minorities facing genocide, like the Uyghurs in China, and journalists investigating powerful drug cartels in Mexico, to communicate safely with each other, knowledgeable sources, and the outside world," ACLU Surveillance and Cybersecurity Counsel Jennifer Granick said in a statement. "There is simply no way for Apple, or any other company, to provide the FBI access to encrypted communications without also providing it to authoritarian foreign governments and weakening our defenses against criminals and hackers."

The government made similar requests following an attack in San Bernadino, California, in 2015. The FBI sued Apple in an attempt to unlock an iPhone belonging to one of the shooters, who police killed in a subsequent shootout. However, the bureau gained access to the device using a third-party tool and dropped the case.

Update (10:56 PM ET): In a statement to the media, Apple pushed back against any suggestion that it has not provided help, claiming "tireless" work in support FBI efforts, and noted its opposition to backdoors that may weaken encryption. Specifically, Apple claimed it had not been informed about a need for additional assistance until January 6th, and there was no subpoena about a second phone until January 8th.

Apple: