US Sen. Bernie Sanders (I-Vermont) at the Communication Workers of America office in Washington on Thursday. REUTERS/Kevin Lamarque The presidential campaign of US Sen. Bernie Sanders (I-Vermont) improperly accessed confidential voter information from the campaign of his Democratic presidential rival Hillary Clinton, according to party officials.

The Washington Post first reported the accusations by the Democratic National Committee on Thursday evening. The incident has "sparked alarm at the DNC," according to The Post, and the organization has cut off the Sanders campaign from its list of likely Democratic voters.

"The decision by the party committee is a major blow to Mr. Sanders's campaign," The New York Times reported. "The database includes information from voters across the nation and is used by campaigns to set strategy, especially in the early voting states."

Sanders' campaign manager, Jeff Weaver, told The Post that a low-level staffer did view Clinton voter information and that one staffer had been fired over the incident. Sanders' campaign spokesman, Michael Briggs, confirmed to The Times that the fired staffer was the same person who viewed the information.

Weaver blamed the illicit viewing on a glitch from software supplied by a DNC-hired vendor, NGP VAN. The DNC also directed blame at NGP VAN.

The Clinton campaign reportedly declined to comment.

The Democratic candidates are set to debate Saturday night, their third meeting this election.