The FBI has asked for Apple’s help in unlocking two iPhones linked to a shooting at a Pensacola naval base in December, according to a report by NBC News. In a letter sent to Apple on Monday, FBI general counsel Dana Boente said the bureau has obtained court approval to search the device, but was unable to unlock the device using available tools.

“Investigators are actively engaging in efforts to ‘guess’ the relevant passcodes but so far have been unsuccessful,” Boente told the company.

“We gave them all of the data in our possession,” says Apple

Such requests have been a sore point for Apple, which cannot produce local data from locked phones without attacking the fundamental elements of iOS security. The company routinely provides iCloud data in response to court orders, which often includes backups of the phone’s local hard drive — but without the password for a specific device, Apple can’t provide data stored locally on the phone. The failure to provide that data has become a sticking point for law enforcement and the FBI in particular, which has often lobbied for increased access to locally encrypted data.

Apple’s failure to decrypt local data last came to a head with the San Bernardino shooting, in which the FBI sought to decrypt a phone linked to another domestic shooting. In particular, the FBI hoped to force Apple to create a modified, encryption-free version of iOS, which would then be signed by Apple and installed on the San Bernardino phone. After months of legal fighting, the FBI abruptly withdrew its case when a vendor solution became available.

When the phone was finally unlocked, it produced no additional leads. A subsequent Inspector General report found that the bureau had failed to explore its internal vendor resources before bringing the case.

Reached by The Verge, Apple declined to confirm the existence of the letter, but said it had turned over all available data related to the case in response to a court order. “We have the greatest respect for law enforcement and have always worked cooperatively to help in their investigations,” a company representative said. “When the FBI requested information from us relating to this case a month ago we gave them all of the data in our possession and we will continue to support them with the data we have available.”