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.
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.”
Tech firms increasingly use products that only the individual user can fully access.
It is not the first time that the FBI director has expressed concerns that the technology designed to protect consumers privacy empowers criminals.
Last year the White House abandoned plans to force tech firms to share encrypted data despite objections from the FBI.
But the issue has assumed greater importance in the wake of the San Bernardino attacks and the deadly jihadist assaults on Paris.
Experts have warned that even if firms provided “back door” routes so law enforcement can access private data, it could also provide a route for hackers.