Federal investigators have still been unable to unlock encrypted data on a cell phone that belonged to one of the attackers involved in the shootings in San Bernardino, California, last year, FBI Director James Comey said Tuesday. Comey made the comments while briefing senators on emerging threats to the United States.

The failure to unlock the phone, which, according to the BBC, belonged to Syed Rizwan Farook, has been hampering the ongoing investigation into the Dec. 2 shooting. Farook and his wife, Tashfeen Malik, opened fire at a holiday party at the Inland Regional Center, killing 14 people. Authorities have been working to retrace Farook and Malik’s movements and their plans before the attack.

The encrypted data in the iPhone could shed light on the communications the couple had with anyone, and also help authorities understand why Farook left a bag with several homemade pipe bombs in the center's conference room.

“We still have one of those killers' phones that we haven’t been able to open," Comey reportedly told the Senate Intelligence Committee. "It has been two months now and we are still working on it.

"It affects cops and prosecutors and sheriffs and detectives trying to make murder cases, kidnapping cases, drugs cases,” he added, according to BBC. "It has an impact on our national security work, but overwhelmingly this is a problem local law enforcement sees."

The FBI, which is investigating the case as an act of terrorism, is working to determine the motive behind the attack. Farook and Malik had pledged allegiance to a leader of the Islamic State group on Facebook hours before the shooting, but authorities have found no evidence that the couple was directly linked to any overseas terrorist group or received any outside support.

Comey reportedly said that encrypted cell phones and text messaging apps have made it difficult for investigators to trace locations and connections once they acquire a suspect's device.