US Attorney General William Barr has asked Apple to unlock two phones of a Saudi man who carried out a mass shooting at a US base.
He accused the company of not having “given us any substantive assistance”, a claim that Apple denies.
The tech firm has clashed previously with the FBI over requests to unlock iPhones belonging to terror suspects.
Three sailors were killed and eight wounded in the 6 December attack by a 21-year old Saudi Air Force lieutenant.
Mr Barr described the shooting at Naval Air Station Pensacola as an “act of terrorism”.
“It is very important for us to know with whom and about what the shooter was communicating before he died,” he said at a press conference on Monday.
Apple had given the FBI iCloud data from the attacker’s online account, the New York Times reported, but refused to unlock the phone, saying it would undermine their own encryption software.
In a statement following Mr Barr’s accusation, Apple rejected that it had not provided assistance with the investigation and said it had responded promptly to multiple FBI requests for information.
A similar clash in 2016 was resolved when the FBI found a way to unlock a phone belonging to a mass shooter in California without help from Apple.