"Fourteen people were slaughtered and many more had their lives and bodies ruined. We owe them a thorough and professional investigation under law. That's what this is," Comey wrote.
Comey wrote in his letter that the FBI wants the chance to try to guess the passcode without the phone self-destructing and without it taking a decade to guess correctly.
"We don't want to break anyone's encryption or set a master key loose on the land," he wrote.
The FBI director said he understood the case highlights the serious tension between privacy and security. He called on the American people to decide how they want to govern themselves in this new world of technological advancements.
"I hope folks will take a deep breath and stop saying the world is ending, but instead use that breath to talk to each other," he said.
Comey wrote that the FBI owes the victims of the San Bernardino attack a thorough and professional investigation that pursues all potential leads.
Some of the families of San Bernardino victims are expected to file an amicus, or friend of the court, brief in support of the government’s argument.
The attorney representing families told ABC News that he was first contacted by the U.S. attorney about filing a brief in the case on Feb. 14, three days before the government sought the court order.
"We believe that the interests of the victims are broader than either a criminal investigation or a criminal prosecution. The victims have questions about why they were targeted and about whether they are still being targeted by other individuals with whom the terrorist may have communicated, and about how something like this could happen in San Bernardino,” said Stephen Larson, a former federal prosecutor who also served as a federal judge.
Magistrate Judge Sheri Pym has imposed a March 3 deadline for amicus filings in the case.