WASHINGTON — The day before the F.B.I. director, James B. Comey, sent a letter to Congress announcing that new evidence had been discovered that might be related to the completed Hillary Clinton email investigation, the Justice Department strongly discouraged the step and told him that he would be breaking with longstanding policy, three law enforcement officials said on Saturday.

Senior Justice Department officials did not move to stop him from sending the letter, officials said, but they did everything short of it, pointing to policies against talking about current criminal investigations or being seen as meddling in elections.

That Mr. Comey moved ahead despite those protestations underscores the unusual nature of Friday’s revelations, which added a dramatic twist to the final days of the presidential campaign. His action reignited a firestorm that Mrs. Clinton believed she had put behind her when the F.B.I. decided in July not to charge anyone in the investigation into the handling of classified information on her private email server.

Mr. Comey’s letter did not reopen that inquiry, even as he promised to review the new information, which was discovered in an unrelated investigation into the disgraced former congressman Anthony D. Weiner.