The Postal Service apologized on Thursday for "inappropriately" releasing a cache of sensitive personal and background information about a congressional candidate.
It also said similar releases from personnel records may have been made in "a small number" of other cases since June.
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The candidate, Abigail Spanberger, previously worked in law enforcement as a postal inspector. The documents included the security clearance form she submitted as part of the application for that job, which her campaign biography says included "working narcotics and money laundering cases." She also served as a CIA officer during her career.
She is a Democrat running to unseat Rep. Dave Brat, a Virginia Republican.
The Postal Service "deeply regrets our mistake," which was a result of "human error," said spokesman Dave Partenheimer.
"We take full responsibility for this unfortunate error, and we have taken immediate steps to ensure this will not happen again," Partenheimer said, including new "guidance to our employees" and follow up training.
A Republican-oriented opposition research group, America Rising Corporation, had requested Spanberger's Official Personnel File under the Freedom of Information Act. That law requires the government to release public records but also requires it to redact several types of information, including personal data. The group then provided the form to the Congressional Leadership Fund, a super PAC supporting Republican House candidates.
Spanberger on Tuesday had accused the two groups of improperly obtaining the document, charges both organizations denied. The Postal Service's admission on Thursday confirms that America Rising Corporation obtained the document through a standard records request.
"The Postal Service has confirmed that they gave information to America Rising and CLF that they never should have provided," Spanberger said in a statement on Thursday.
"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," she added.
The National Archive's National Personnel Records Center said it received a FOIA request for the records and followed its policy of passing the request along to the Postal Service.
Spokeswoman Sandra Austin told CNN on Wednesday the request did not include an authorization from Spanberger to release her full file. Without an authorization, the NPRC typically only releases a bare-bones Transcript of Service that lists positions and dates of employment.