The method used by the FBI to open the iPhone of San Bernardino terrorist Syed Farook is unlikely to help police open hundreds of other iPhones that may contain evidence of a crime, according to government officials and legal experts.
On Monday, the Justice Department said it "successfully accessed the data stored on Farook's iPhone," using a method devised by an outside third party.
"We are now able to unlock that iPhone without compromising any information on the phone," said Eileen Decker, the U.S. Attorney in Los Angeles.
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But for both technical and legal reasons, the FBI is unlikely to use the same method to unlock iPhones and other electronic devices sitting unopened in state and local crime labs around the nation.
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On Wednesday, for example, prosecutors in Arkansas said they've asked the FBI to help open an iPhone 6 and an iPod belonging to defendants in a murder case. A judge earlier this week agreed to delay the trial to give investigators more time to try to unlock the devices.
But federal officials familiar with the technique that was employed to open the San Bernardino phone say it likely works only on an iPhone 5c, the model Farook was using, running a specific version of the iOS 9 operating system. It hasn't been tried on other models yet.
That word is a disappointment to officials at the office of New York City District Attorney Cyrus Vance. They said Thursday that their investigators have 215 iPhones they want to open for evidence, but none are 5c models.
In another high profile case, prosecutors in Baton Rouge, Louisiana believe a locked iPhone may hold important clues in the murder of 29-year-old Brittney Mills, who was shot and killed last year when she opened her apartment door.
That device, however, is an iPhone 5s running iOS 8.2.
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Still, District Attorney Hillar Moore said it's worth asking for help.
"I've reached out to the FBI and I'm waiting to hear back," he told NBC News on Thursday.
But even if the method used to open Farook's phone would work on other models, there are legal reasons why it's unlikely to be used in pending state and local criminal cases.
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It would have to be certified as a legally valid forensic tool, in order to have the contents of an unlocked iPhone admissible as evidence in court. That would take time to document.
And if the method was used in a criminal trial, defense attorneys would have the right to ask how it worked, under recent U.S. Supreme Court decisions involving physical evidence.
"A defendant has a right to confront the technical expert about how the relevant evidence was obtained," according to Professor Steve Vladeck of the Washington College of Law at American University.
"I think that would give the government an awful lot of pause to expose the technique to cross examination."
Federal officials have said the FBI does not want to disclose the method and have suggested that it has been classified as a government secret.