This mea culpa of sorts comes just a few days after the FBI said it hadn't been a mistake.

"As I understand from the experts, there was a mistake made in that 24 hours after the attack where the [San Bernardino] county at the FBI's request took steps that made it hard—impossible—later to cause the phone to back up again to the iCloud," Comey said during a congressional hearing on Tuesday.

But when the Department of Justice first revealed that Farook's iCloud password had been reset by the San Bernardino County Department of Public Health, the FBI released a statement essentially denying that changing the password was a mistake.

Comey appeared in front of the House Judiciary Committee on Tuesday for a hearing on the FBI's fight with Apple over accessing the phone of the San Bernardino shooter, who killed 14 people in a rampage on December 5 of last year. The FBI has recently requested Apple to disable some security features on the phone, which would allow the bureau's agents to guess the passcode to unlock the shooter's iPhone 5C. Apple has fought the request in the court, as well as with an unusual media push, arguing that this case would set a dangerous precedent.