The FBI has all but concluded that there’s nothing to tell Apple about a vulnerability that was exploited to unlock a phone left behind by San Bernardino terrorist Syed Farook, federal officials said Tuesday, because investigators don’t know what it was.

Using an idea proposed by a so-far unidentified third party, the FBI was able to defeat the phone’s security features earlier this month and extract data stored on the device.

Normally, when the federal government discovers vulnerabilities in devices or software, it notifies the companies involved.