After the DNC penalized the presidential campaign of Bernie Sanders for improperly accessing data collected by rival Hillary Clinton, campaign officials came out against the DNC decision and threatened to pursue legal action if their access to the database is not restored. (Reuters)

Officials with the Democratic National Committee have accused the presidential campaign of Sen. Bernie Sanders of improperly accessing confidential voter information gathered by the rival campaign of Hillary Clinton, according to several party officials.

Jeff Weaver, the Vermont senator’s campaign manager, acknowledged that a staffer had viewed the information but blamed a software vendor hired by the DNC for a glitch that allowed access. Weaver said one Sanders staffer was fired over the incident.

The discovery sparked alarm at the DNC, which promptly shut off the Sanders campaign’s access to the strategically crucial list of likely Democratic voters.

The DNC maintains the master list and rents it to national and state campaigns, which then add their own, proprietary information gathered by field workers and volunteers. Firewalls are supposed to prevent campaigns from viewing data gathered by their rivals.

NGP VAN, the vendor that handles the master file, said the incident occurred Wednesday while a patch was being applied to the software. The process briefly opened a window into proprietary information from other campaigns, said the company’s chief, Stu Trevelyan. He said a full audit will be conducted.

The DNC has told the Sanders campaign that it will not be allowed access to the data again until it provides an explanation as well as assurances that all Clinton data has been destroyed.

Having his campaign cut off from the national party’s voter data is a strategic setback for Sanders — and could be a devastating blow if it lasts. The episode also raises questions about the DNC’s ability to provide strategic resources to campaigns and state parties.

Sanders spokesman Michael Briggs said four Sanders campaign staffers accessed Clinton data, and that three of them did so at the direction of their boss, Josh Uretsky, who was the operative fired.

Uretsky told CNN Friday morning that he and others on the campaign discovered the software glitch Wednesday morning and probed the system to discover the extent of their own data’s exposure. He said there was no attempt to take Clinton information but said he took responsibility for the situation.

“We investigated it for a short period of time to see the scope of the Sanders campaign’s exposure and then the breach was shut down presumably by the vendor,” he told CNN. “We did not gain any material benefit.”

Weaver said the Sanders campaign never downloaded or printed any of the data, meaning it is no longer in possession of any proprietary information. He squarely blamed NGP VAN for the glitch — and blamed the DNC for hiring the company.

He said the campaign has flagged similar problems with the software for the DNC in the past.

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“Sadly, the DNC is relying on an incompetent vendor who on more than one occasion has dropped the firewall between the various Democratic candidates’ data,” he said.

The DNC is likely to initiate an outside audit to examine how the incident occurred and whether any other data was improperly seen.

“The DNC places a high priority on maintaining the security of our system and protecting the data on it,” DNC spokesman Luis Miranda said in a statement. “We are working with our campaigns and the vendor to have full clarity on the extent of the breach, ensure that this isolated incident does not happen again, and to enable our campaigns to continue engaging voters on the issues that matter most to them and their families.”

Brian Fallon, a spokesman for the Clinton campaign, declined to comment.

Trevelyan, the head of NGP VAN, described the episode as an “isolated incident that was fairly short in duration,” and he added: “By lunchtime, it was resolved.”

Officials said they were unlikely to ask for a criminal investigation. However, Sanders could face political fallout from the impression that his staff worked to gain an unfair advantage.

Voter information is the lifeblood of day-to-day campaign contact with potential supporters. Sophisticated campaigns spend millions gathering information such as how likely individual voters are to cast ballots in primary elections as well as which candidate they currently favor.

The data also could include detailed consumer information designed to help the campaign target their appeals to groups of voters.

The dispute erupted as Sanders and Clinton prepare to take part in their third debate, in New Hampshire on Saturday night.

Maintaining the master list is one of the prime responsibilities of the national party committee, particularly at a time when other outside political groups and super PACs are increasingly taking on roles once served by party committees.

For that reason, the security lapse could deal a blow to the DNC. Already, in Republican politics, outside organizations have been working to compile master voter lists independently from the Republican National Committee.

This story was first published Thursday at 11:47 p.m.

Abby Phillip contributed to this report.