The F.B.I. interviewed Hillary Clinton on Saturday morning for its investigation into whether she or her aides broke the law by corresponding through a private email server set up for her use as secretary of state, a controversy that has dogged her presidential campaign and provided fodder for her political rivals.

The voluntary interview, which took place over three and a half hours at the F.B.I. headquarters in Washington, largely focused on the Justice Department’s central question: Did the actions of Mrs. Clinton or her staff rise to the level of criminal mishandling of classified information?

It could take weeks or longer to reach a decision, but news that Mrs. Clinton, the Democratic Party’s presumptive nominee, had been questioned in the J. Edgar Hoover Building three weeks before her party’s convention quickly reverberated.

The Republican National Committee called the step “unprecedented,” while Mrs. Clinton’s expected opponent in the race for the White House, Donald J. Trump, wasted little time before weighing in.