WASHINGTON — James B. Comey, the former F.B.I. director, said on Thursday that President Barack Obama’s attorney general sought in 2015 to play down the significance of an investigation into Hillary Clinton’s use of a private email server.

In his wide-ranging testimony to the Senate Intelligence Committee, Mr. Comey said the request by that attorney general, Loretta E. Lynch, led him to question her independence.

Last summer, Mr. Comey said the F.B.I. was completing its investigation into Mrs. Clinton, and he took it upon himself to announce that “no reasonable prosecutor” would bring charges against her, after concluding that Ms. Lynch might be perceived as politically compromised. One of the incidents he cited was a request by Ms. Lynch to use a blander term to publicly describe the investigation in the midst of a presidential election.

“The attorney general had directed me not to call it an investigation, but instead to call it a matter, which confused me and concerned me,” Mr. Comey said in response to questions during testimony, which focused mainly on President Trump’s decision to fire him last month. “That was one of the bricks in the load that led me to conclude, ‘I have to step away from the department if we’re to close this case credibly.’”