Investigators obtained Syed Rizwan Farook's work phone with an authorized search warrant, but "has been unable to complete the search because it cannot access the phone’s encrypted content,” federal prosecutors said in a court filing.
“Apple has the exclusive technical means which would assist the government in completing its search, but has declined to provide that assistance voluntarily,” prosecutors said.
Authorities hope to obtain “crucial evidence” on the phone about the terror attacks at the Inland Regional Center in San Bernardino, California on Dec. 2, the filing states.
Investigators hope to gain insight on who Farook and his wife, Tashfeen Malik, may have contacted in plotting the attack. They are also interested to learn where the couple may have traveled to before and after the shooting, along with any other "pertinent" information.
The phone is owned by Farook's employer, the San Bernardino County Department of Public Health. The department has given authorities consent to search the phone, but it's locked with a numeric password.
The FBI's attempts to crack the passcode have failed because Apple has set its phone systems with a function that automatically erases the access key and renders the phone "permanently inaccessible" after 10 failed attempts.
Investigators don't know if Farook enabled that function, but they are concerned that the phone may erase all of its contents.
Prosecutors insist that Apple has the ability to modify the software and ensure that the auto-erase function is turned off.
"This would allow the government multiple investigative attempts to determine the passcode in a timely manner, without fear that the data subject to search under the warrant would be rendered permanently inaccessible," a U.S. magistrate wrote.
A representative for Apple did not immediately return ABC News' request for comment.
Farook and Malik stormed the Inland Regional Center during a Department of Public Health training session and holiday party in December, killing 14 people and injuring 22. Nearly all of those who were killed or injured worked for the county.