NEW YORK (Reuters) - The Federal Bureau of Investigation interviewed Democratic U.S. presidential candidate Hillary Clinton for three and a half hours on Saturday as part of the probe into her use of a private email server while serving as secretary of state, her campaign said. The interview at FBI headquarters in Washington followed a week of intense public focus on the investigation and on  Clinton’s viability as a presidential candidate, with four months to go to the election. Her campaign has tried for months to downplay the controversy as a distraction.   In an interview broadcast on MSNBC, Clinton said she was happy to do the FBI interview, which her spokesman earlier described as “voluntary.”  “I’ve been answering questions for over a year” regarding the private email server, Clinton said. It was not clear if the questioning of Clinton signaled an imminent conclusion to the investigation in a pivotal time for the presidential race. It does follow FBI interviews of several of Clinton’s former staff members, as well as her top aide Huma Abedin.  Clinton is expected to be formally nominated as the Democratic candidate for the Nov. 8 presidential election at the party’s convention in less than four weeks.  “Timing of FBI interview, between primaries and convention, probably good timing for @HillaryClinton,” tweeted David Axelrod, a former senior adviser to President Barack Obama who served as the chief strategist for his two presidential campaigns. “Best to get it behind her.” Clinton is currently the front-runner for the White House with polls showing her leading presumptive Republican nominee Donald Trump. In a tweet on Saturday, Trump said it was “impossible for the FBI not to recommend criminal charges against Hillary Clinton. What she did was wrong!”   He also criticized Clinton’s husband, former President Bill Clinton, for meeting privately with Attorney General Loretta Lynch earlier this week, which Lynch later said she regretted, though she said they did not discuss the investigation.  “What Bill did was stupid!” Trump tweeted. The FBI is investigating whether anyone in Clinton’s operation broke the law as result of a personal email server kept in her Chappaqua, New York, home while she was secretary of state from 2009 to 2013. One of the questions is whether they mishandled classified information on the server.   Clinton and her staff have struggled to respond to accusations that her use of the server in violation of State Department protocol means she is untrustworthy. Trump has said the investigation should disqualify her from being president. “@HillaryClinton campaign statement says She “voluntarily” met w @FBI for 3.5 hours this morning - yeah, lots of people volunteer 2 do that,” tweeted the Republican National Committee’s communications director, Sean Spicer. FBI Director James Comey said in testimony to Congress in March he felt pressure to complete the investigation quickly. Adding to the uncertainty over Clinton is the FBI’s refusal to say who is the target of its investigation. Republican lawmakers have called for an independent investigation, saying they do not trust the Justice Department to handle the inquiry with impartiality. Republicans, including Trump, intensified their criticism of the process on Thursday after news emerged of Lynch’s meeting with Bill Clinton.  Lynch said she would accept whatever recommendations the career prosecutors working on the case made about whether to prosecute Clinton. The FBI probe and the partisan fight over the server have added an extra layer of uncertainty to one of the most tumultuous presidential races in recent memory. Trump, a political novice once dismissed by the Republican establishment, will likely emerge this month as the party’s nominee and has set his sights on Clinton, who he has labeled “Crooked Hillary.”  Clinton held a nine-point lead over Trump in a Reuters/Ipsos poll released on Friday. 