Several US government sources say the FBI paid under US$1 million for the technique used to unlock the iPhone used by one of the San Bernardino shooters.
The figure is smaller than the US$1.3 million the agency's chief initially indicated the hack would cost.
The Federal Bureau of Investigation will be able to use the technique to unlock other iPhone 5C models running iOS 9 -- the specifications of the shooter's phone -- without additional payment to the contractor who provided it, the sources added on Thursday (local time).
FBI Director James Comey last week said the agency paid more to get into the iPhone than he will make in the remaining seven years and four months he has in his job, suggesting the hack cost over $1.3 billion, based on his annual salary.
The Justice Department unlocked the iPhone in March with the help of the contractor after Apple Inc refused to bypass the device's encryption features on grounds it could undermine security for all users.
The FBI, not the contractor, has physical possession of the mechanism used to open the phone but does not know details of how it works, one of the sources said.
The identity of the contractor is so closely-held inside the FBI that not even Comey knows who it is, one of the sources said.
The FBI is still examining the contents of the iPhone in the hope they will help fill gaps in their investigation of the shooting last December which killed 14 people and wounded 22, two US government sources.
Investigators are looking into whether shooters Syed Rizwan Farook and his wife Tashfeen Malik, had associates or co-conspirators, the sources said.