The U.S. Postal Service took responsibility Thursday for the unprecedented release of a security clearance application from an ex-CIA officer–and indicated that what it called a “mistake” may not be an isolated incident.

The Postal Service, in its first substantial comment on the issue, offered no explanation for the disturbing release other than unspecified “human error.”

The application belongs to Abigail Spanberger, who is retired from the agency and running for Congress as a Democrat in Virginia. Spanberger’s application, which contains extraordinarily revealing personal information as a matter of course, ended up in the hands of a Republican-aligned opposition research firm and then a super PAC tied to House Speaker Paul Ryan. It marks the first time a security clearance application became fodder for a domestic political attack.

“The Postal Service deeply regrets our mistake in inappropriately releasing Ms. Spanberger’s Official Personnel File (‘OPF’) to a third-party, which occurred because of human error. We take full responsibility for this unfortunate error, and we have taken immediate steps to ensure this will not happen again,” Postal Service spokesperson David Partenheimer said in a statement provided to The Daily Beast and other news outlets.

The statement tacitly refutes Spanberger’s allegation that the GOP-tied firms acquired her clearance application through nefarious means.

Dozens of clearance holders are currently running for political office. Two others, ex-CIA and Pentagon official Elissa Slotkin and retired Marine lieutenant colonel Amy McGrath, told The Daily Beast that the release of Spanberger’s SF-86 is chilling. While neither possessed evidence that their applications had been released, Slotkin feared for the confidentiality of her own.

The Postal Service’s Partenheimer indicated there was cause for concern.

“We are continuing our review, but believe the issue began in June of 2018, and that only a small number of additional requests for information from personnel files were improperly processed,” Partenheimer said in the statement.

Partenheimer said he is not answering follow-up questions about how many federal officials had their personnel files “processed” and whether the Postal Service released those files and to whom, referring to an ongoing internal review.

According to BuzzFeed and the PAC, the research group acquired Spanberger’s clearance application, known as an SF-86, through a Freedom of Information Act (FOIA) request filed with a division of the National Archives. That division reportedly referred the FOIA to the U.S. Postal Service, which the PAC said delivered Spanberger’s SF-86 in its unredacted form.

All of that raises a host of questions, since such a highly sensitive document–designed to root out potential blackmail targets or agents of foreign powers–is exempt from FOIA. Its release is very likely a violation of the Privacy Act. Those interested in an SF-86, according to security officials, are typically hostile foreign powers, not rival domestic political organizations.

The Postal Service comment was its first since The New York Times broke Spanberger’s story on Tuesday night. On Wednesday, the only explanation the Postal Service offered was that it was looking into the SF-86 release. The Daily Beast, on Tuesday night, filed its own FOIA request with the service for records documenting the SF-86 request and fulfillment.

Partenheimer said in his statement that the Postal Service has now provided “clear instructions and guidance” for relevant employees and pledged additional training and safeguards. Partenheimer offered neither specifics nor timetables.

“The privacy and security of personal information is of utmost importance to the Postal Service. The Postal Service offers our sincere apology to Ms. Spanberger, and we will request the return of the information which we mistakenly disclosed,” he said.

In a statement, Spanberger said she was “review[ing] potential legal remedies” against the Postal Service, the opposition research firm America Rising and the super PAC Congressional Leadership Fund (CLF). She also said she was “awaiting confirmation” from the Postal Service that her SF-86 was definitively among her personnel documents disclosed by the Postal Service, where she used to work.

"It is my sincere hope that USPS will provide significantly more detail as to how this major failure occurred, and that CLF and America Rising will put decency and country before politics and comply with USPS’s request that they return all documents received. I also expect that CLF will comply with our original cease and desist and stop sharing my personal identifying information, including my Social Security Number,” Spanberger said.