Hillary Clinton said she was “eager” to be interviewed by the FBI this weekend regarding the agency’s investigation into her use of a private email server while secretary of state, though she declined to say who said the setup was legal and whether it violated government regulations regarding the handling of classified material.

Clinton, the presumptive Democratic presidential nominee, was interviewed Saturday morning at FBI headquarters in Washington, D.C.  

Her campaign said the interview was voluntary and lasted three-and-a-half hours.

“It was something I had offered to do since last August,” Clinton told MSNBC in a phone interview Saturday evening. “I've been eager to do it, and I was pleased to have the opportunity to assist the department in bringing its review to a conclusion.”

Nevertheless, Clinton said she has no knowledge about when the FBI will conclude its case before sending its recommendation to Attorney General Loretta Lynch.

Clinton also defended her use of the private server by repeating that she “never received nor sent any material that was marked classified,” while acknowledging some of the emails have been “retroactively" re-classified.

When asked who advised her about the server, Clinton said: “I'm not going to go into any more detail then I already have in public many times … out of respect for the process that the department is conducting.”

The server controversy, which has cast a shadow over Clinton’s campaign since the start, intensified Thursday when her husband, former President Bill Clinton, had an impromptu meeting with Lynch, on the tarmac of a Phoenix airport.

Clinton told MSNBC that Lynch and her husband’s exchange was a “short, chance meeting” in which the Justice Department review was not discussed. But she seemed to acknowledge that the meeting was not a good idea, saying “hindsight is 20/20.”

Presumptive Republican presidential nominee Donald Trump on Saturday called for charges to be filed against Clinton in the case.

"It is impossible for the FBI not to recommend criminal charges against Hillary Clinton," Trump tweeted. "What she did was wrong! What Bill did was stupid!"

Neither the FBI nor Justice Department has commented on the interview.

Republican National Committee Chairman Reince Priebus said: “Hillary Clinton has just taken the unprecedented step of becoming the first major party presidential candidate to be interviewed by the FBI as part of a criminal investigation surrounding her reckless conduct.”

He also said the Lynch-Clinton encounter “raises serious concerns about special treatment.”

There was already speculation about whether an agency under the Obama administration could conduct an unbiased probe, which only intensified after Clinton met with Lynch, a President Obama appointee who decides whether to bring charges in the case.  

Lynch says she "fully expects" to accept whatever recommendations she receives from the agency's career prosecutors and lawyers. She also acknowledged the meeting with the former president has “cast a shadow” over the investigation.

Hillary Clinton has said relying on a private server was a mistake but that other secretaries of state had also used personal email addresses.

The matter was referred for investigation last July by the inspectors general for the State Department and intelligence community following the discovery of emails that they said contained classified information.

The State Department's inspector general, the agency's internal watchdog, said in a blistering audit in May that Clinton and her team ignored clear warnings from State Department officials that her email setup violated federal standards and could leave sensitive material vulnerable to hackers. Clinton declined to talk to the inspector general, but the audit reported that Clinton feared "the personal being accessible" if she used a government email account.

Agents have already interviewed top Clinton aides including her former State Department chief of staff Cheryl Mills and Huma Abedin, a longtime aide who is currently the vice chairwoman of Clinton's campaign.

The staffer who set up the server, Bryan Pagliano, was granted limited immunity from prosecution by the Justice Department last fall in exchange for his cooperation. The FBI as a matter of course seeks to interview individuals central to an investigation before concluding its work.

The emails were routed through a server located in the basement of Clinton's New York home during her tenure as the nation's top diplomat.

The Associated Press contributed to this report.