“We have a tool that works on a narrow slice of phones,” Mr Comey said at a conference on encryption and surveillance in Ohio.
Mr Comey added that the technique would not work on the iPhone 5s and the later models iPhone 6 and 6s.
The Justice Department said in March it had unlocked the San Bernardino shooter’s iPhone with the help of an unidentified third party and dropped its case against Apple.
As the technique cannot be used to break into newer models, law enforcement authorities will likely have to lean on Apple to help them access devices in other cases.
The Justice Department has asked a New York court to force Apple to unlock an iPhone 5s related to a drug investigation.
Prosecutors in that case said they would update the court by April 11 on whether it would “modify” its request. If the government continues to pursue that case, the company could potentially use legal discovery to force the FBI to reveal what technique it used