Brett Kavanaugh, Donald Trump’s conservative nominee for the supreme court, has denied a sexual misconduct allegation from when he was in high school.

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

Kavanaugh’s statement comes after Senator Dianne Feinstein, the top Democrat on the Senate judiciary committee, said she has notified federal investigators about information she received about the nominee but won’t disclose publicly.

The New Yorker reported the alleged incident took place at a party when Kavanaugh, now 53, was attending Georgetown preparatory school. The woman making the allegation attended a nearby school.

The magazine says the woman sent a letter about the allegation to Democrats. A Democratic aide and another person familiar with the letter confirmed Friday to the Associated Press that the allegation is sexual in nature. Two other people familiar with the matter confirmed to the AP that the alleged incident happened in high school. They were not authorized to speak publicly and spoke on condition of anonymity.

The AP has not confirmed the details of the alleged incident in the New Yorker’s account.

Rallying to Kavanaugh’s defense, 65 women who knew him in high school issued a letter, released by Republicans on the Senate judiciary committee, saying he has “always treated women with decency and respect”.

“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,” wrote the women, who said most of them had attended all-girl high schools in the area. “For the entire time we have known Brett Kavanaugh, he has behaved honorably and treated women with respect.”

The judiciary committee, which has finished confirmation hearings for Kavanagh, is scheduled to vote next Thursday on whether to recommend that he be confirmed by the full Senate.

The White House called Feinstein’s move an “11th hour attempt to delay his confirmation”.

The California Democrat said in a statement Thursday that she “received information from an individual concerning the nomination”. She said the person “strongly requested confidentiality, declined to come forward or press the matter further, and I have honored that decision”.

The FBI confirmed that it received the information Wednesday evening and included it in Kavanaugh’s background file, which is maintained as part of his nomination. The agency said that is its standard process.

Feinstein’s statement that she has “referred the matter to federal investigative authorities” jolted Capitol Hill and threatened to disrupt what has been a steady path toward confirmation for Kavanaugh by Republicans eager to see the conservative judge on the court.

Feinstein has held the letter close. Democratic senators on the panel met privately Wednesday evening and discussed the information, according to Senate aides who were not authorized to discuss the situation publicly and spoke on condition of anonymity.

A spokeswoman for Anna Eshoo, a Democratic representative from California, declined to confirm reports that the congresswoman had forwarded a letter containing the allegations to Feinstein. She said her office has a confidentiality policy regarding casework for constituents.

A White House spokeswoman, Kerri Kupec, said the FBI has vetted Kavanaugh “thoroughly and repeatedly” during his career in government and the judiciary.

She said Kavanaugh has had 65 meetings with senators – including with Feinstein – sat through over 30 hours of testimony and publicly addressed more than 2,000 questions. “Not until the eve of his confirmation has Sen. Feinstein or anyone raised the specter of new ’information’ about him”, she said.

John Cornyn, the second-ranking Republican Senator from Texas and a member of the committee, was also skeptical.

“Let me get this straight: this is (sic) statement about secret letter regarding a secret matter and an unidentified person. Right,” he tweeted.

Kavanaugh has undergone six federal background checks over his time in government, including one most recently for the nomination, the aide said.

The new information on Kavanaugh was included Thursday in his confidential background file at the committee and is now available for senators to review, the aide said.

Democrats don’t have the votes to block Kavanaugh’s nomination if Republicans are unified in favor of it.