Bruce Sewell, senior vice president and general counsel for Apple Inc., is sworn in before testifying to the House Judiciary hearing on "The Encryption Tightrope: Balancing Americans' Security and Privacy" on Capitol Hill in Washington March 1, 2016. [Photo/Agencies]

Comey responded that the Federal Bureau of Investigation was not asking to expand the government's surveillance authority, but rather to maintain its ability to obtain electronic information under legal authorities that Congress has already provided.

He also acknowledged that it was a "mistake" for the FBI to have asked San Bernardino County officials to reset the phone's cloud storage account after it was seized. The decision prevented the device, which was owned by the county, Farook's employer, from backing up information that the FBI could have read.

Farook and his wife, Tashfeen Malik, shot and killed 14 people and wounded 22 others last Dec. 2 before they were themselves killed in a shootout with police. The government has said the attack was inspired by Islamist militants and the FBI wants to read the phone's data to investigate any links with militant groups.

Comey told a congressional panel last Thursday that the phone could have "locator services" that would help the agency fill in a gap in its knowledge of the route the couple traveled as they fled.

"We're missing 19 minutes before they were finally killed by law enforcement," Comey said. "The answer to that might be on the device."

A federal judge handed Apple a victory in another phone unlocking case in Brooklyn on Monday, ruling that he did not have the legal authority to order Apple to disable the security of an iPhone that was seized during a drug investigation.

US Attorney General Loretta Lynch said on Tuesday at the RSA Cybersecurity conference in San Francisco that she was "disappointed" by the Brooklyn ruling, and rebuffed Apple's claim that its Fifth Amendment protection against self-incrimination was being violated.

The Justice Department is "not alleging that [Apple has] done anything wrong," Lynch said, but is treating the company as a third party holding data valuable to an ongoing investigation.

Manhattan District Attorney Cyrus Vance testified in support of the FBI on Tuesday, arguing that default device encryption "severely harms" criminal prosecutions at the state level, including in cases in his district involving at least 175 iPhones.