On Thursday, Senate Democrats disclosed that they had referred a complaint regarding President Trump’s Supreme Court nominee, Judge Brett Kavanaugh, to the F.B.I. for investigation. The complaint came from a woman who accused Kavanaugh of sexual misconduct when they were both in high school, more than thirty years ago.

The woman, who has asked not to be identified, first approached Democratic lawmakers in July, shortly after Trump nominated Kavanaugh. The allegation dates back to the early nineteen-eighties, when Kavanaugh was a high-school student at Georgetown Preparatory School, in Bethesda, Maryland, and the woman attended a nearby high school. In the letter, the woman alleged that, during an encounter at a party, Kavanaugh held her down, and that he attempted to force himself on her. She claimed in the letter that Kavanaugh and a classmate of his, both of whom had been drinking, turned up music that was playing in the room to conceal the sound of her protests, and that Kavanaugh covered her mouth with his hand. She was able to free herself. Although the alleged incident took place decades ago and the three individuals involved were minors, the woman said that the memory had been a source of ongoing distress for her, and that she had sought psychological treatment as a result.

In a statement, Kavanaugh said, “I categorically and unequivocally deny this allegation. I did not do this back in high school or at any time.”

Kavanaugh’s classmate said of the woman’s allegation, “I have no recollection of that.”

The woman declined a request for an interview.

In recent months, the woman had told friends that Kavanaugh’s nomination had revived the pain of the memory, and that she was grappling with whether to go public with her story. She contacted her congresswoman, Anna Eshoo, a Democrat, sending her a letter describing her allegation. (When reached for comment, a spokesperson for Eshoo’s office cited a confidentiality policy regarding constituent services and declined to comment further on the matter.)

The letter was also sent to the office of Senator Dianne Feinstein. As the ranking minority member of the Senate Judiciary Committee, Feinstein was preparing to lead Democratic questioning of Kavanaugh during his confirmation hearing weeks later. The woman contacted Feinstein’s office directly, according to multiple sources.

After the interactions with Eshoo’s and Feinstein’s offices, the woman decided not to speak about the matter publicly. She had repeatedly reported the allegation to members of Congress and, watching Kavanaugh move toward what looked like an increasingly assured confirmation, she decided to end her effort to come forward, a source close to the woman said. Feinstein’s office did not respond to requests for comment.

Feinstein’s decision to handle the matter in her own office, without notifying other members of the Senate Judiciary Committee, stirred concern among her Democratic colleagues. For several days, Feinstein declined requests from other Democrats on the Judiciary Committee to share the woman’s letter and other relevant communications. A source familiar with the committee’s activities said that Feinstein’s staff initially conveyed to other Democratic members’ offices that the incident was too distant in the past to merit public discussion, and that Feinstein had “taken care of it.” On Wednesday, after media inquiries to the Democratic members multiplied, and concern among congressional colleagues increased, Feinstein agreed to brief the other Democrats on the committee, with no staff present.

On Thursday, Feinstein announced that she had referred the matter to the F.B.I. “I have received information from an individual concerning the nomination of Brett Kavanaugh to the Supreme Court,” Feinstein said. “That individual strongly requested confidentiality, declined to come forward or press the matter further, and I have honored that decision. I have, however, referred the matter to federal investigative authorities.”

In a statement, an F.B.I. spokesperson said, “Upon receipt of the information on the night of September 12, we included it as part of Judge Kavanaugh’s background file, as per the standard process.”

After Feinstein’s announcement, a White House spokesperson, Kerri Kupec, wrote, about Kavanaugh, “Not until the eve of his confirmation has Sen. Feinstein or anyone raised the specter of new ‘information’ about him,” calling it an “11th hour attempt to delay his confirmation.”

Given the heightened attention to issues of sexual misconduct amid the #MeToo movement, the political risks of mishandling the allegation were acute, particularly for Feinstein, who is up for reëlection this year and is facing a challenge from her left. During Clarence Thomas’s Supreme Court confirmation hearing, in 1991, the Senate was accused by some of failing to take seriously enough Anita Hill’s allegations that Thomas had sexually harassed her while acting as her boss at the Equal Employment Opportunity Commission. After the Thomas hearings concluded, it emerged that Senator Joe Biden, who was the Democratic chairman of the Judiciary Committee at the time, had failed to call three additional women to the witness stand who had been willing to offer testimony confirming Hill’s complaints about Thomas’s inappropriate behavior toward women. Last December, Biden, who may run for President in 2020, publicly apologized for failing Hill, saying, “I wish I had been able to do more.”

Sources familiar with Feinstein’s decision suggested that she was acting out of concern for the privacy of the accuser, knowing that the woman would be subject to fierce partisan attacks if she came forward. Feinstein also acted out of a sense that Democrats would be better off focussing on legal, rather than personal, issues in their questioning of Kavanaugh. Sources who worked for other members of the Judiciary Committee said that they respected the need to protect the woman’s privacy, but that they didn’t understand why Feinstein had resisted answering legitimate questions about the allegation. “We couldn’t understand what their rationale is for not briefing members on this. This is all very weird,” one of the congressional sources said. Another added, “She’s had the letter since late July. And we all just found out about it.”