Sen. Bernie Sanders' campaign manager, Jeff Weaver, speaks during a news conference at the Bernie 2016 Campaign Office in Washington. AP Photo/Manuel Balce Ceneta The campaign of Democratic presidential candidate and Sen. Bernie Sanders (I-Vermont) accused the national Democratic Party of working to "actively undermine" its campaign in a hastily scheduled press conference on Friday.

Campaign manager Jeff Weaver threatened to sue the Democratic National Committee after it revoked Sanders' access to its database of voters. The DNC did that after Sanders' team improperly accessed data compiled by the campaign of top rival Hillary Clinton.

"This is taking our campaign hostage," Weaver said in a press conference in Washington, a day before the next Democratic presidential debate.

Later in the day, the campaign made good on that threat: The Sanders campaign filed a lawsuit in federal court to regain access to the voter data. In the suit, the Sanders campaign alleged that the suspension was costing the campaign at least $600,000 per day. The lawsuit also alleges that the DNC did not provide the Sanders campaign with notice or opportunity consistent with an earlier agreement between the campaigns and the committee.

A summary of computer logs showed that four aides to Sanders' campaign accessed voter data compiled by Clinton's campaign, according to The Associated Press. Some of the aides reportedly saved the information. The Clinton campaign said its data was breached by the Sanders campaign in "25 searches by four different accounts," and that the data was saved into the Sanders campaign's account.

"This breach is totally unacceptable and may have been a violation of the law," Clinton campaign manager Robby Mook said in a conference call with reporters Friday evening.

"We are particularly disturbed right now that they are using the fact that they stole data as a reason to raise money for their campaign," he added. "And I would certainly hope that they would stop trying to make money off of what they did, stop politicizing and work to make sure that what took place is remedied."

The Sanders campaign said it had fired a staffer at center of the breach incident — data director Josh Uretsky.

Weaver blamed a vendor hired by the committee — political-data company NGP VAN — for what he alleged was continued "dangerous incompetence." He said he was confident that months ago, some of the Sanders campaign's own data was lost to another campaign.

"First, as I pointed out earlier, this is not the first time that the vendor hired by the DNC … has allowed serious failures to occur. On more than one occasion they have dropped the firewall between the data of competing Democratic campaigns. This is dangerous incompetence. It was our campaign months ago that alerted the DNC to the fact that the campaign data was being made available to other campaigns," he said.

US Democratic presidential candidate and US Senator Bernie Sanders speaks at a campaign stop in Conway, New Hampshire, August 24, 2015. REUTERS/Brian Snyder He added: "Once again, the sensitive and important data was compromise because the DNC and its vendor failed to protect it. We have invested enormous campaign resources in acquiring the rights to use this proprietary data. But the DNC, in an inappropriate overreaction, has denied us access to our very own data. This is data collected by hundreds of thousands of volunteers across America."

Uretsky told CNN Friday that he was only trying to "understand how badly the Sanders campaign's data was exposed" and not attempting to take any of the Clinton campaign's data.

Weaver said that after the campaign alerted the Democratic National Committee to a previous breach, it "became clear" that the Sanders campaign had accessed other campaigns' data. He said the campaign was looking into whether other staffers were involved and that "further disciplinary action may follow."

Then he accused the committee of using the incident to choke off the "lifeblood" of the Sanders campaign to boost Clinton's bid.

"Third, rather incredibly, the leadership of the DNC has used this incident to shut down the ability to access our own information — information which is the lifeblood of this campaign," Weaver said.

He continued:

By their action, the leadership of the Democratic National Committee is now actively attempting to undermine our campaign. This is unacceptable. Individual leaders of the DNC can support Hillary Clinton in any way they want. But they are not going to sabotage our campaign, one of the strongest grassroots campaigns in modern history. We are announcing today that if the DNC continues to hold our data hostage and continues to try to attack the heart and soul of our grassroots campaign, we will be in federal court this afternoon seeking immediate relief.

Bernie Sanders and Hillary Clinton. AP

DNC Chair and Rep. Debbie Wasserman Schultz (D-Florida) fired back with a statement while Weaver was speaking.

"Once the DNC became aware that the Sanders campaign had inappropriately and systematically accessed Clinton campaign data, and in doing so violated the agreement that all the presidential campaigns have signed with the DNC, as the agreement provides, we directed NGP VAN to suspend the Sanders campaign's access to the system until the DNC is provided with a full accounting of whether or not this information was used and the way in which it was disposed," she said.

She added: "I have personally reached out to Sen. Sanders to make sure that he is aware of the situation. When we receive this report from the Sanders campaign, we will make a determination on re-enabling the campaign's access to the system."

Brian Fallon, the Clinton campaign's national press secretary, said in a statement later Friday afternoon calling on the Sanders campaign and DNC to "work expeditiously ensure that our data is not in the Sanders campaign's account and that the Sanders campaign only have access to their own data.”

Mook said later Friday that he could "unequivocally tell you that no member of our campaign ever reached into Sanders’ data and extracted it in the way that they did this week."

This post has been updated with comment from the Clinton campaign and details about the Sanders campaign's lawsuit.

Dan Turkel contributed reporting.