The FBI is seeking permission to file a second, secret declaration in US District Court in Washington, DC describing its search for documents related to the bureau's probe of Hillary Clinton's use of a private email server during her tenure as secretary of state.

The declaration is to be submitted in response to a Freedom of Information Act (FOIA) lawsuit filed against the bureau by VICE News seeking the contents of Clinton's email server and the FBI's correspondence with the State Department. It contains "additional details" about the ongoing investigation, according to a court filing submitted to US District Court Judge Randolph Moss Monday night asking him to accept the secret document, which came on the eve of major Democratic primary contests in California, New Jersey, and four other states,

The details of the declaration pertain, in part, to the bureau's search for documents about Clinton's server and how the public release of the records could harm the FBI's investigation, Justice Department lawyers wrote in a court filing, seeking dismissal of VICE News' lawsuit.

"These details supplement defendant's showing that it conducted a reasonable search, but cannot be disclosed on the public record without compromising information that the FBI seeks to protect," the filing said.

Attorneys representing VICE News had objected to the government's filing of a classified declaration last March, saying the government failed to ask the court's permission to file the "ex parte" declaration and provide advance notice to them.

Clinton's exclusive use of private email to conduct official business was revealed a year ago this month by the New York Times. The State Department released more than 30,000 of Clinton's emails over the course of 10 months under a court order in response to a separate FOIA lawsuit filed by VICE News in January 2015.

The secret declaration was written by FBI FOIA chief David Hardy, who also wrote the classified one last March. Hardy also wrote a new, public declaration that was submitted to the court Monday. In it, Hardy said the bureau cannot even disclose the "to, from, dates and their contents" in "two pieces of correspondence from the FBI to the Department of State," nor could the FBI reveal the "total volume of responsive information" about Clinton's server without compromising the probe.

'This is an active and ongoing investigation.'

Government attorneys also steadfastly refused to characterize the nature of the FBI's probe into Clinton's use of a private email server as criminal. In the court filing, attorneys would only say that the probe is based on a "security referral" from the Inspectors General of the Intelligence Community and the Department of State, and "made for counterintelligence purposes."

Attorneys Ryan James and Jeffrey Light, who are handling the FOIA lawsuit on behalf of VICE News, argued in a motion filed last month that the FBI has not provided enough information to justify withholding entire documents related to Clinton's email server.

The FBI "should be able to identify a particular individual or a particular incident as the object of its investigation and the connection between that individual or incident and a possible security risk or violation of federal law," James and Light wrote. "The FBI has not cited, at least in its public filings, to any 'statutes whose violation could reasonably have been thought evidenced by' Ms. Clinton's use of a private email server."

Publicly stating that the FBI's investigation is criminal in nature would satisfy the withholding of records compiled for law enforcement purposes under FOIA, James and Light argued. But in Monday's court filing, government attorneys said "the FBI is not required to identify a particular federal statute that it alleges has been violated in connection with the pending investigation, or the target(s) of the investigation, to meet the ... threshold" for withholding documents compiled for law enforcement purposes.

In the latest court filing, government attorneys would only say that the FBI "is working on a referral from the Inspectors General of the Intelligence Community and the Department of State in connection with former Secretary of State Hillary Clinton's use of a private e-mail server." The referral was "a security referral made for counterintelligence purposes," and the investigation is "being conducted under the FBI's assigned law enforcement authorities."

In a separate, public declaration filed by Hardy in March, he said any documents the FBI retrieved from Clinton's email server, which would be responsive to VICE News' FOIA request, "are potential evidence in the FBI's investigation, or may provide leads to or context for potential evidence" and are exempt from disclosure under FOIA because they would be deemed law enforcement records.

"As this is an active and ongoing investigation, the FBI is continuing to assess the evidentiary value of any materials retrieved for the investigation from any such server equipment/related devices," the filing said. "Disclosure of evidence, potential evidence, or information that has not yet been assessed for evidentiary value while the investigation is active and ongoing could reasonably be expected to undermine the pending investigation by prematurely revealing its scope and focus."

In a footnote to the new declaration filed Monday night, Hardy said, "Due to the sensitive nature of the investigation, the number of FBI personnel and involved in and having knowledge of the pending investigation is limited."

That disclosure contradicts a report about Clinton's email woes published last March by the Washington Post that claimed "one hundred forty-seven FBI agents have been deployed to run down leads" in the case. The Post later corrected that number to "fewer than 50."

The Associated Press declared on Monday night that Clinton has secured enough delegates to clinch the Democratic nomination, setting up a general election showdown with presumptive Republican nominee Donald Trump in November.

This report has been updated to reflect that the government is seeking permission from the judge presiding over the case to file the secret declaration.