Syed Farook and his wife Tashfeen Malik killed 14 people in the Islamic State-inspired attack in California.
A mobile phone belonging to Farook was recovered but FBI Director James Comey said encryption technology meant they had not been able to access it.
Such technology was "overwhelmingly" affecting law enforcement, he warned.
Mr Comey made the comments at a Senate Intelligence Committee hearing.
"It affects cops and prosecutors and sheriffs and detectives trying to make murder cases, kidnapping cases, drugs cases.
"It has an impact on our national security work but overwhelmingly this is a problem local law enforcement sees."
This story goes to the heart of the law enforcement vs Silicon Valley debate.
How can it be right, police ask, that the phone of a terrorist can't be accessed by police?
It is an argument FBI chief James Comey has been making for months now, and he argues that encrypted devices are hindering police investigations.
But Silicon Valley's defence has always been the same. If you provide a way for law enforcement to access a criminal's phone once it has been locked, they say, then you're also opening the door to hackers.
All phones - yours, mine, everyone's - would be inherently less secure.
Follow Dave Lee on Twitter @DaveLeeBBC and on Facebook
US investigators are still unable to unlock a phone owned by one of the attackers involved in the shootings in San Bernardino last year, the FBI says.