California Sen. Dianne Feinstein referred a letter about Supreme Court nominee Brett Kavanaugh to the FBI.

The letter reportedly details an incident when Kavanaugh was in high school.

A woman alleges that in the early 1980s, when Kavanaugh was in high school, he attempted to force himself on her.

Kavanaugh denied the allegation.

The FBI added the letter to Kavanaugh's background file.

WASHINGTON, DC — New information emerged on Friday about the letter Sen. Dianne Feinstein referred to federal investigators regarding Brett Kavanaugh, President Donald Trump's Supreme Court nominee.

Feinstein had sent the letter to the FBI on Thursday after having kept its content secret, even from fellow Democrats on the Senate Judiciary Committee. It detailed an allegation against Kavanaugh while he was a teenager in the Washington, DC, area.

While the letter itself was known of publicly for several days, The New Yorker revealed what it said. The author of the letter, an unidentified woman, claims that while at a high-school party as teenagers, Kavanaugh pinned the woman down and forced himself on her. The woman claims that a male friend of Kavanaugh's turned up music volume in the room to drown out any noise, according to The New Yorker.

Kavanaugh denied the allegation in a statement, and a classmate of his told The New Yorker he had "no recollection" of it occurring.

"I categorically and unequivocally deny this allegation," Kavanaugh said. "I did not do this back in high school or at any time."

Feinstein had previously withheld the letter after receiving it from California Rep. Anna Eshoo, concealing its contents from other members of the committee, including Democrats.

After Feinstein briefed Democrats on the Judiciary Committee, she sent the letter to federal investigators and publicly acknowledged doing so in a statement. The FBI then added the letter to Kavanaugh's background file.

The White House condemned the secrecy of the letter as a last ditch attempt to derail Kavanaugh's nomination.

"Throughout 25 years of public service, the Federal Bureau of Investigation has thoroughly and repeatedly vetted Judge Kavanaugh, dating back to 1993, for some of the most highly sensitive roles. He has served in the Office of Independent Counsel, the White House, and on the D.C. Circuit Court of Appeals, all before his nomination earlier this year to serve as Associate Justice on the Supreme Court," White House spokeswoman Kerri Kupec said on Thursday.

"Senator Schumer promised to 'oppose Judge Kavanaugh's nomination with everything I have,' and it appears he is delivering with this 11th-hour attempt to delay his confirmation," Kupec said.

Later on Friday, Senate Judiciary Chairman Chuck Grassley announced that the committee would be holding its procedural vote to advance Kavanaugh out of the committee. A full confirmation vote is expected in October.