The FBI said thousands of encrypted phones are hindering its investigations, but the agency is refusing to back up its claims.

For years, the agency said that encryption helps criminals evade justice because investigators and prosecutors are locked out from accessing intelligence that they say can help convictions. The agency calls this "going dark," and it has for years argued that tech companies should undermine encryption by installing backdoors that give police access, much to the chagrin of experts.

The FBI will occasionally say how many phones and devices it can't unlock. At the last count earlier this year, it was 7,775 encrypted devices -- a reportedly inflated figure.

But the FBI has never said how many investigations suffer as a result. In doing so, the agency would quantify the scale of the "going dark" problem.

ZDNet filed a request under the Freedom of Information Act (FOIA) to find out.

In a letter Monday, the FBI denied the request, saying the information was exempt from disclosure, as the records "could reasonably be expected to interfere with enforcement proceedings."

The FBI also would not say if the records do or do not exist.

"It's worthless to know that the FBI can't get into any number of phones if 99 percent of those phones weren't important to making their case," said Robyn Greene, policy counsel and government affairs lead at New America's Open Technology Institute, told ZDNet.

"Members of Congress who are buying what the FBI is selling are just taking it on faith that there's actually a problem -- even the FBI doesn't know its extent, if there is one at all," she said.

We filed the FOIA request in October, when the FBI revealed it had 6,900 encrypted devices that it couldn't unlock. In our request, we asked for "all records and documents relating to the 6,900 mobile devices" that the agency had in custody.

We also asked for the FBI's internal tracking data for the past five years, which former FBI director James Comey revealed in testimony before Congress in 2015 that the agency was developing. He said the FBI was at the time "working to identify new mechanisms to better capture and convey the challenges encountered with lawful access to both data-in-motion and data-at-rest."

Sen. Chuck Grassley requested that once the FBI had that information for it to be shared with Congress, though it's not known if the agency ever did.

A spokesperson for Grassley's office did not immediately return a request for comment.

We also asked the FBI if that data collection system had been implemented some three years later, but we did not hear back.

It's not the first records request made since the FBI disclosed its "unhackable" 7,775 devices figure.

The Electronic Frontier Foundation said last week in a blog post that the group is "asking for proof" of the FBI's claim that it has thousands of supposedly unhackable phones.

In a far more detailed and granular records request, the privacy group is asking the FBI and other agencies for more information about how it reached the 7,775 figure and its current capabilities in unlocking devices.

You can read our letter from the FBI below.