WASHINGTON (Reuters) – FBI Director James Comey said on Tuesday that federal investigators have still been unable to access the contents of a cellphone belonging to one of the killers in the Dec. 2 shootings in San Bernardino, California, due to encryption technology.

Comey told the Senate Intelligence Committee that the phenomenon of communications “going dark” due to more sophisticated technology and wider use of encryption is “overwhelmingly affecting” law enforcement operations, including investigations into murder, car accidents, drug trafficking and the proliferation of child pornography.

“We still have one of those killer’s phones that we have not been able to open,” Comey said in reference to the San Bernardino attack.

Syed Rizwan Farook, 28, launched the Islamic State-inspired attack with his wife, Tashfeen Malik, 29, at a social services agency in the California city, leaving 14 dead.

Comey and other federal officials have long warned that powerful encryption poses a challenge for criminal and national security investigators, though the FBI director added Tuesday that “overwhelmingly this is a problem that local law enforcement sees.”

Technology experts and privacy advocates counter that so-called “back door” access provided to authorities would expose data to malicious actors and undermine the overall security of the Internet.

A study from the Berkman Center for Internet and Society at Harvard released last month citing some current and former intelligence officials concluded that fears about encryption are overstated in part because new technologies have given investigators unprecedented means to track suspects.

Senator Ron Wyden, an Oregon Democrat, asked Director of National Intelligence James Clapper to provide a declassified response to the Berkman study within 60 days. Clapper agreed to the request.

The White House last year abandoned a push for legislation that would mandate U.S. technology firms to allow investigators a way to overcome encryption protections, amid rigorous private sector opposition. But the issue has found renewed life after the shootings in San Bernardino and Paris.

Senators Richard Burr and Dianne Feinstein, the Republican and Democratic leaders of the intelligence panel, have said they would like to pursue encryption legislation, though neither has introduced a bill yet.

(Reporting by Dustin Volz and Mark Hosenball; editing by Sandra Maler and G Crosse)