Law enforcement officials were able to break into the phone used by Syed Farook with the help of an unnamed third party.
"The FBI has now successfully retrieved the data stored on the San Bernardino terrorist's iPhone and therefore no longer requires the assistance from Apple required by this Court Order," DOJ spokeswoman Melanie Newman said in a statement.
Farook was one of two shooters in the December 2015 attack that left 14 people dead, and the government has been trying to force Apple to help bypass the phone's security features.
Apple has declined to help, saying it would compromise the security of all iPhone users. The company has argued that law enforcement officials don't understand the consequences of creating a backdoor and the cybersecurity dangers that it poses.
The two sides were due in court last week, but the judge granted a last minute request from DOJ to postpone the hearing, saying an unidentified "outside party" came to the FBI with an alternative method for hacking into the phone.
The fact that the DOJ was able to successfully access the phone without Apple's help is bad news for the tech giant because it suggests Apple's encryption technology is not as advanced as once thought.
On Monday, the Department of Justice said the method only works on this particular phone, which is an iPhone 5C running a version of iOS 9 software.
A law enforcement official, speaking to reporters on condition of anonymity, would not reveal how it pulled off this hack. He would not name the "third party" that helped the FBI. And he refused to say whether the FBI will disclose this hacking method to Apple so the company can protect future phones from being hacked this way.
"We can't comment on the possibility of future disclosures at this point," the law enforcement official said.