FBI technicians have been trying and failing to break the encryption of a phone used by the couple who killed 14 people in a terrorist attack in San Bernardino, Calif. in December, according to FBI Director James Comey.

"We still have one of those killers' phones that we haven't been able to open," said Comey at a hearing of the Senate Intelligence Committee this week, as reported by the Los Angeles Times. "It has been two months now and we are still working on it."

Though the exact kind of phone used wasn't mentioned during the hearing, the difficulty encountered by the FBI would suggest a relatively recent iPhone or Android device with support for full-disk encryption. Once a passcode is enabled, such devices can be essentially impossible for outside parties to break into. Apple itself has claimed that it can't crack an iPhone with iOS 8 or 9, even when served with a warrant.

Comey repeated his stance that encrypted phones and messaging services are making it increasingly tough to pursue some investigations and surveillance. He denied that he wanted companies to offer backdoors, but nevertheless insisted that they should be able to offer data access when presented with a court order.

"I don't want a back door... I would like people to comply with court orders, and that is the conversation I am trying to have," he explained.