(Bloomberg) -- Bernie Sanders’ presidential campaign threatened to challenge the Democratic National Committee in federal court on Friday for "actively undermining" its work following an incident that led to the firing of its national data director.

Sanders staffers exploited a temporary glitch in the DNC's voter database to save lists created by Hillary Clinton’s campaign, according to an audit of the breach obtained by Bloomberg. In response, Sanders' team was banned from accessing all of the DNC's data as well as its own data, a move that is "taking our campaign hostage," campaign manager Jeff Weaver said at a press conference outside Sanders's Capitol Hill campaign office.

Weaver sought to downplay the severity of the incident as he spoke briefly to reporters, saying that the campaign had not used the Clinton data and placing blame on the party and its vendor, NGP VAN, for letting security lapses persist. He also said that NGP VAN has told the campaign that no Clinton data was printed or downloaded, despite audit logs.

"We are running a clean campaign. We are going to beat Secretary Clinton," he said. "We don’t need dirty tricks."

Staffer missteps reflect that “in the heat of these campaigns sometimes young people make misjudgments,” Weaver added.

Weaver also suggested that other campaigns may have had access to Sanders data earlier this fall and said his campaign had notified the DNC of potential breaches at the time.

In a statement released as Weaver spoke, DNC chairwoman Debbie Wasserman Schultz said Sanders staffers had "inappropriately and systematically access" Clinton data, running afoul of an agreement that all three Democratic presidential campaigns have signed with the party.

The Sanders team's explanation of the incident has gradually changed since it was first confirmed late Thursday, initially saying that only a single “low-level” staffer accessed the Clinton data and that none of it was saved.

But the database logs created by NGP VAN show that four accounts associated with the Sanders team took advantage of the Wednesday morning breach. Staffers conducted searches that would be especially advantageous to the campaign, including lists of its likeliest supporters in 10 early voting states, including Iowa and New Hampshire. Campaigns rent access to a master file of DNC voter information the party, and update the files with their own data culled from field work and other investments.

After one Sanders account gained access to the Clinton data, the audits show, that user began sharing permissions with other Sanders users. The staffers who secured access to the Clinton data included national data director Josh Uretsky, who was fired on Thursday, and his deputy, Russell Drapkin. The two other usernames that viewed Clinton information were “talani" and "csmith_bernie," created by Uretsky's account after the breach began.

Though the Sanders campaign initially claimed that it had not saved Clinton data, the logs show that the Vermont senator’s team created at least 24 lists during the 40-minute breach, which started at 10:40 a.m., and saved those lists to their personal folders. The Sanders searches included New Hampshire lists related to older voters, "HFA Turnout 60-100" and "HFA Support 50-100," that were conducted and saved by Uretsky. Drapkin's account searched for and saved lists including "HFA Support <30" in Iowa and "HFA Turnout 30-70"' in New Hampshire.

Uretsky told CNN on Friday that he probed the Clinton data to get a sense of the full extent of the problem, but the breach was reported to NGP VAN by a third party, not by the Sanders campaign, a source familiar with the investigation said. "We knew there was a security breach in the data, and we were just trying to understand it and what was happening," he said. "To the best of my knowledge, nobody took anything that would have given the (Sanders) campaign any benefit."

The Sanders campaign confirmed late Thursday that it had fired a staffer who had accessed modeling data from the Clinton campaign but did not at the time identify the aide, though two sources told Bloomberg it was Uretsky. "That behavior is unacceptable and that staffer was immediately fired," Sanders communications director Michael Briggs said in a statement.

Sanders was having one of the best days of his presidential campaign Thursday before the incident came to light, racking up his largest union endorsement yet, from the Communications Workers of America, as well as the backing of liberal group Democracy for America. His campaign also announced Thursday that it had surpassed 2 million contributions since its launch in late April. But by the end of the day, it had fired Uretsky.

Briggs said the blame lies with NGP VAN's system. "Sadly, the vendor who runs the DNC's voter file program continues to make serious errors," he said, noting the campaign had alerted the DNC to problems months ago.

NGP VAN's CEO, Stu Trevelyan, said the breach, first reported by the Washington Post, was an “isolated incident" after it released new code on Wednesday that contained a bug.

After the firm identified the bug, it began an audit and determined that "only one campaign took actions that could possibly have led to it retaining data to which it should not have had access," Trevelyan said in a statement.

His firm, he added, played no role in suspending the Sanders campaign's access to data and contractually could not do so. "Moving forward, we are adding to our safeguards around these issues," he said.

The DNC has asked NGP VAN to conduct an analysis to identify users who accessed other campaigns' data and the actions they took, and to conduct a full audit of its system as well and to begin a review process with all the campaigns and staffers use it.

"The DNC places a high priority on maintaining the security of our system and protecting the data on it," communications director Luis Miranda said. "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."

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