(CNN) The FBI has asked Apple to help investigators access two iPhones believed to be owned by a 21-year-old man who killed three sailors in a shooting last month at a Pensacola Air Force base, a person familiar with the investigation said.

Investigators believe Mohammed Alshamrani, a second lieutenant in the Royal Saudi Air Force and a student naval flight officer, left one phone in the car and was carrying the second phone while he fired on individuals at the base. He shot that phone, the person said. Alshamrani was killed after exchanging gunfire with deputies on scene.

The FBI has investigated the shooting as an act of terror but has stopped short of formally designating it as such. The Justice Department is expected to conclude that the shooting was terrorism, the source said.

On Monday, the FBI sent Apple a letter, requesting assistance to bypass the passcodes on the phones, the person said.

A spokeswoman for the FBI confirmed the agency had sent a letter to Apple on Monday, but declined to comment further on its contents. NBC News first reported the FBI's request.

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