Supreme Court nominee Brett Kavanaugh vigorously denied claims involving an alleged high school incident made in an undisclosed letter and turned over to FBI by Sen. Dianne Feinstein, D-Calif.

“I categorically and unequivocally deny this allegation. I did not do this back in high school or at any time,” Kavanaugh, 53, said in a statement Friday.

Feinstein ignited controversy Thursday by releasing a statement saying she turned information about Kavanaugh over to the FBI. She did not detail the accusation, and Republicans accused her of trying to orchestrate a last-minute smear.

“I have received information from an individual concerning the nomination of Brett Kavanaugh to the Supreme Court,” Feinstein said in her surprise statement. “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.”

Fox News confirmed that the letter involved an allegation about Kavanaugh while a student at Georgetown Preparatory School in Bethesda, Maryland in the 1980s. A woman, who was also in high school at the time, accused Kavanaugh in the letter of holding her down and trying to force himself on her during a party, before she got away. The details were first reported Friday by the New Yorker.

The woman also claimed Kavanaugh was joined at the time by a friend who turned up music to conceal her protests. But that unnamed classmate reportedly told the New Yorker, “I have no recollection of that.”

Meanwhile, the Judiciary Committee on Friday received a letter from 65 women who said they knew Kavanaugh from high school and vouched for him as a “good person.” The letter was addressed to Feinstein and Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa.

“We are women who have known Brett Kavanaugh for more than 35 years and knew him while he attended high school between 1979 and 1983,” the letter, obtained by Fox News, reads. “For the entire time we have known Brett Kavanaugh, he has behaved honorably and treated women with respect. We strongly believe it is important to convey this information to the committee at this time.”

The White House blasted the charge on Thursday as a last-minute gambit.

“Not until the eve of his confirmation has Sen. Feinstein or anyone raised the specter of new ‘information’ about him," White House spokesperson Kerri Kupec said in a statement.

The accusation comes after the Senate Judiciary Committee already grilled Kavanaugh and other witnesses and prepares to vote on sending his nomination to the full Senate.

A source familiar with the confirmation proceedings told Fox News that Feinstein received the letter back in July, but did not make its existence known publicly until Thursday.

The letter reportedly was given to Feinstein by Rep. Anna Eshoo, D-Calif., but has not been publicly disclosed by senators who have seen the document.

Feinstein met privately with Kavanaugh on August 20 and also questioned him repeatedly in open and closed session during the Judiciary Committee hearings on his nomination last week. There is no indication that the matter came up in either the private meeting or the closed committee session.

The FBI conducts background checks on all major government appointees, including Supreme Court nominees.

“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,” the FBI said in a statement. Fox News has learned that the White House would have to request that the bureau follow up on the letter for the matter to be investigated further. It was not clear whether the White House had done so as of Thursday evening.

The woman referenced in the letter has yet to be identified, but is being represented by Debra Katz, a whistleblower attorney who works with #MeToo survivors, according to The Intercept.

Despite the turmoil over the letter, a spokesperson for Grassley said there is no plan to delay Kavanaugh’s confirmation.

Grassley set the panel's vote on Kavanaugh for Sept. 20 and Republicans hope to confirm Kavanaugh by the start of the new court session Oct. 1.

Fox News' Jake Gibson, Mike Emanuel and John Roberts contributed to this report.