Last week, the F.B.I. successfully broke into an iPhone belonging to one of the terrorists in last year’s San Bernardino shootings, ending the government’s contentious, public legal dispute with Apple. While the agency won’t disclose its method for unlocking the device, it has sent a memo to local law-enforcement agencies, telling them it can provide technical assistance to help solve other cases where encrypted Apple devices could contain evidence. “In mid-March, an outside party demonstrated to the FBI a possible method for unlocking the iPhone,” the letter obtained by BuzzFeed News reads. “That method for unlocking that specific iPhone proved successful.” The letter was sent in response to questions from local law-enforcement agents about the F.B.I.’s method for unlocking the iPhone. “We are in this together,” it continues.

The Washington Post had previously reported that the F.B.I. was weighing whether it would share its method to break into San Bernardino shooter Syed Farook’s iPhone with local agencies. Last week, the federal-law-enforcement agency agreed to help a prosecutor unlock two Apple devices in a double-murder case. The devices belonged to two teenagers who are accused of killing an elderly couple in Arkansas. Prosecuting attorney Cody Hiland and the Conway Police Department had submitted a request to the F.B.I. to unlock the devices, which the F.B.I. agreed to on Wednesday afternoon. The memo obtained by BuzzFeed News suggests the F.B.I. may be somewhat limited in its capacity to help local law enforcement, since its method for unlocking the iPhone is classified.

Apple engineers and other experts say the F.B.I.’s method for unlocking the iPhone is likely to leak sooner rather than later, Reuters reports, allowing Apple to fix whatever flaw in its encryption enabled the F.B.I. and a third party to unlock the iPhone. The company is expected to announce when the vulnerability has been eliminated, potentially extending its public showdown with the U.S. government.

It’s still unclear who helped the F.B.I. crack into Farook’s iPhone. Unconfirmed reports suggest it had help in the San Bernardino case from Israeli mobile forensics firm Cellebrite, which specializes in data extraction and has publicly stated its ability to hack into Apple devices. Cellebrite has received more than $2 million in purchase orders from the F.B.I. in the past seven years.