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James Comey, the man in charge of the FBI, has admitted that the tool they used to crack the San Bernardino terrorist’s iPhone is very limited in capability. The tool, provided by a third party, works on only a small subset of devices, specifically, only on the model used by the attacker – the iPhone 5c running iOS 9.

“This worked to open this one 5c iOS 9 phone, but it’s a bit of a technological corner case. The world has moved on. This doesn’t work in 6 or 5s. So we have a tool that works on a narrow slice of iPhones,” Comey told an audience at Kenyon College.

Comey went on to say that the FBI is still deliberating on whether to inform Apple of what specific exploit the tool used to gain access the phone. “If we tell Apple, they are going to fix it and we are back where we started from,” he said.

Since opening the phone, the FBI offered the tool to other US law enforcement and intelligence agencies.

“As has been our longstanding policy, the FBI will of course consider any tool that might be helpful to our partners,” said a letter the FBI sent to those other agencies.

“We know that the absence of lawful, critical investigative tools due to the ‘Going Dark’ problem is a substantial state and local law enforcement challenge that you face daily,” the letter said, according to a copy obtained by Buzzfeed.

However, due to the limited nature of the tool, it has now been cast into doubt how useful it will be to other agencies.