Abigail Spanberger, Democratic candidate for the 7th congressional district of Virginia, speaks during the Women's Summit in Herndon, Va., on Saturday June 23, 2018.

The United States Postal Service said Thursday that "human error" led to it releasing a former CIA operative and Democratic House candidate's highly sensitive security clearance form.

In a statement to CNBC, Postal Service spokesman David Partenheimer said the agency "deeply regrets our mistake in inappropriately releasing" Abigail Spanberger's official personnel file, which contains sensitive personal information. The Postal Service apologized to Spanberger, a candidate running in the highly competitive Virginia 7th District House race, and said it would ask for her information to be returned.

The Congressional Leadership Fund, a House GOP-linked super PAC, used the file for political attacks against Spanberger. The America Rising PAC, a pro-GOP research group, asked for Spanberger's personnel file under a Freedom of Information Act request and received it legally. She spent two years at the Postal Inspection Service before the CIA officially brought her on, according to The New York Times.

In a statement, America Rising CEO Joe Pounder said that the organization "never published [Spanberger's] personal information and has no interest in it." He said America rising is "glad to return the documents" for their redaction.

Partenheimer said the Postal Service would change its process for handling document requests in the future.

"We take full responsibility for this unfortunate error, and we have taken immediate steps to ensure this will not happen again. The Postal Service has addressed the issue by providing clear instructions and guidance to our employees tasked with the responsibility for handling these requests, and we will follow up with additional training," he said.

"The Postal Service also intends to change our process for handling requests for OPF information to provide further protection against its inadvertent release, and to ensure that such requests are properly handled," Partenheimer added.

In a statement Thursday, Spanberger said that "when I served as a federal agent with the US Postal Inspection Service and as a case officer with the Central Intelligence Agency, I did so to serve my country and protect my fellow citizens." The Democratic candidate said she is "profoundly disappointed" by the PAC's move to share what she said is an "unredacted" national security questionnaire, "a document that could not have been legally provided to them."

She went on to accuse the groups of circulating private data, including her Social Security number and medical history, despite cease-and-desist letters sent by her campaign. Spanberger added that she wanted the USPS to "provide significantly more detail" about what happened, and urged the pro-GOP groups to return the documents.

On Tuesday, CLF criticized Spanberger over her time working as a substitute teacher at an Islamic school in Northern Virginia. The CIA had conditionally given her a job at that time as it looked into her background. Spanberger eventually spent eight years as an intelligence officer.

In a statement, CLF communications director Courtney Alexander called Spanberger's public concern about the documents "a dishonest effort to cover up the reality that she hasn't been transparent about her work history."

Spanberger is trying to unseat Republican Rep. Dave Brat in a race that top forecasters consider highly competitive.