FBI agents have confirmed that they recovered records from Hillary Clinton's private server and other electronic devices, but won't reveal any details about those records because that information is being treated as "evidence" in a "pending investigation."

"[A]ll of the materials retrieved from any electronic equipment obtained from former Secretary Clinton for the investigation are evidence, potential evidence, or information that has not yet been assessed for evidentiary value," the FBI said in documents filed with a federal court Monday.

While media reports last year suggested the law enforcement agency had some success pulling potentially deleted emails off Clinton's private server, which it seized in August, the FBI had previously shied away from confirming the existence of recovered records. It also hadn't revealed any details about what was being considered "evidence" in its high-profile investigation.

"Due to the sensitive nature of the investigation, the number of FBI personnel involved in and having knowledge of the pending investigation is limited," the FBI said in the filings.

The FBI argued that disclosing even the number of records it has pulled off Clinton's server, thumb drive or other unnamed devices "could reasonably lead to the public identification and compromising of potential witnesses, as well as defensive actions to conceal activities, elude detection, and/or suppress or fabricate evidence."

The details were revealed through filings in a Freedom of Information Act lawsuit filed by Vice News against the FBI in search of records related to its investigation of Clinton's private email use.

While the FBI asked a federal judge to allow it to keep information about its probe secret, including a pair of emails sent between FBI and State Department officials, the law enforcement agency did reveal its investigation is focused on a "security referral made for counterintelligence purposes" by the inspector general of the intelligence community.

That detail was the latest to undermine Clinton's repeated argument that the investigation is a routine "security review," and suggests the FBI is looking into the specific mishandling of classified intelligence.

In March 2015, Clinton claimed she had deleted roughly 30,000 emails off her private server because she had deemed them personal in nature. Her legal team submitted another 30,000 "work-related" messages to the State Department in December 2014.

But the FBI's most recent admission may cast further doubt on her claim that every government communication was included in the batch of records she surrendered in 2014.