FBI discovers a way to hack into iPhone.

The FBI's discovery of a way to hack into the phone of one of the San Bernardino killers may not be the master key that allows prosecutors across the country to unlock iPhones in hundreds of more ordinary criminal cases.

ALSO READ : FBI Has Accessed San Bernardino Shooter's iPhone Without Apple's Help.

The FBI may not quickly share the technique it used with local law enforcement agencies, New York City Police Commissioner William Bratton said Tuesday. And even if it does, the hack may be too expensive for district attorneys' offices, Manhattan District Attorney Cyrus R. Vance Jr. has said. Also, technology experts said it is not at all certain whether the technique can work with other types of iPhones.

While the San Bernardino case involved an extremist attack December 2 that killed 14 people, investigators across the US are seeking access to iPhones in drug cases and other crimes, arguing that encryption features prevent them from gathering valuable information such as the identity of the person a victim last talked to or texted.