WASHINGTON — The FBI is facing an increasing struggle to access readable information and evidence from digital devices because of default encryption, a senior FBI official told members of Congress at a hearing on digital encryption Tuesday.

Amy Hess said officials encountered passwords in 30 percent of the phones the FBI seized in the last six months, and investigators have had “no capability” to access information in about 13 percent of the cases.

“We have seen those numbers continue to increase, and clearly that presents us with a challenge,” said Hess, the executive director of the FBI branch that oversees the development of surveillance technologies.

In her testimony to a subcommittee of the House Energy and Commerce Committee, Hess defended the Justice Department’s use of a still-unidentified third party to break into the locked iPhone used by one of the two San Bernardino, California, attackers.

But she said the reliance on an outside entity represented just “one potential solution” and that there’s no “one-size-fits-all” approach for recovering evidence.

“These solutions are very case-by-case specific,” she said.

“They may not work in all instances. They’re very dependent upon the fragilities of the system, the vulnerabilities we might find,” she said.

She added that cooperation between the government, academia and private industry was needed to come up with more solutions.

Asked about the FBI’s reliance on a third party to get into the phone, and its inability to access the device on its own, Hess said the work requires “a lot of highly skilled specialized resources that we may not have immediately available to us.”

Representatives from local law enforcement agencies echoed Hess’s concerns.

The hearing comes amid an ongoing dispute between law enforcement and Silicon Valley about how to balance consumer privacy against the need for police and federal agents to recover communications.

Bruce Sewell, Apple’s general counsel, touted the importance of encryption in light of devastating breaches of sensitive government information – including at the IRS and the Office of Personnel Management.