The U.S. Postal Service has confirmed it improperly included Virginia Democratic candidate Abigail Spanberger’s highly confidential federal security clearance application in a package to a conservative opposition research group in response to a Freedom of Information Act request.

The mistake occurred “because of human error,” Dave Partenheimer, a public affairs specialist for the USPS, said in a statement Thursday.

“We take full responsibility for this unfortunate error, and we have taken immediate steps to ensure this will not happen again,” Partenheimer said.

America Rising, the conservative opposition group that filed the FOIA request, passed along the package with Spanberger’s SF-86 security clearance form to the Congressional Leadership Fund, the super PAC closely affiliated with House GOP leadership and Speaker Paul D. Ryan.

Spanberger, who is running against GOP Rep. Dave Brat in Virginia’s hotly contested 7th District, had indicated Tuesday and Wednesday that she believed CLF and America Rising obtained the form through illegal channels.

“I am not aware of any legal way that CLF could have this document,” Spanberger, a former CIA officer and U.S. Postal Inspection Service officer, wrote in a cease-and-desist letter to CLF for circulating the form to at least one reporter at The Associated Press.

CLF’s lawyers issued a response cease-and-desist letter of their own Wednesday afternoon, threatening legal action against Spanberger for “leveling false and irresponsible charges” about how CLF obtained the SF-86.

After the USPS released its statement accepting responsibility for the release of Spanberger’s information, CLF said Spanberger owes the group an apology.

“For the last 48 hours Abigail Spanberger and her liberal allies in Washington have engaged in a misleading smear campaign against our organization and now that the truth has come out she owes us an apology,” CLF communications director Courtney Alexander said.

But Spanberger said in a statement that after speaking with the USPS’ chief postal inspector, “many unanswered questions remain.”

“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,” Spanberger said.

America Rising CEO said in a statement he would be “glad” to return the documents to the USPS “for their redaction.”

Spanberger filed an SF-86 as part of her application for a security clearance when she was applying to work for the U.S. Postal Inspection Service in the mid-2000s.

SF-86s contain highly sensitive personal information — such as family and medical history and the applicant’s social security number — and are supposed to be tightly locked away under protection of the 1974 Privacy Act.

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

The USPS said it has issued “clear instructions and guidance” to its employees tasked with processing FOIA requests.

Spanberger wasn’t the only victim of the USPS’s oversight, Partenheimer indicated. A “small number of additional requests for information from personnel files were improperly processed,” he said.