A day after it was reported that Sen. Dianne Feinstein (D-CA) possesses a sensitive document about Supreme Court nominee Brett Kavanaugh, the senator says she is referring the matter to the Justice Department.

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

The document in question is shrouded with mystery. And its existence only became public after a cryptic report in The Intercept on Wednesday. According to the publication, Democrats on the Senate Judiciary Committee had grown frustrated by Feinstein’s refusal to share the contents of the document with her colleagues.

The document is believed to be a letter detailing an interaction between an unnamed woman and Kavanaugh dating back to their time together in high school. The New York Times reported on Thursday that it involved possible sexual misconduct.

The letter was initially sent to the office of Rep. Anna Eshoo (D-CA), who represents Palo Alto, home to Stanford University. According to The Intercept, the incident involving Kavanaugh was “relayed to someone affiliated with Stanford University, who authored the letter.”

Congressional sources told The Daily Beast that Eshoo received the letter after Kavanaugh was formally nominated. She then passed it on to Feinstein, the top Democrat on the judiciary committee.

White House spokeswoman Kerri Kupec called Feinstein’s referral an “11th hour attempt to delay [Kavanaugh’s] confirmation” and one that would reveal nothing new.

“Throughout his confirmation process, Judge Kavanaugh has had 65 meetings with senators—including with Senator Feinstein—sat through over 30 hours of testimony, addressed over 2,000 questions in a public setting and additional questions in a confidential session,” Kupec said. “Not until the eve of his confirmation has Sen. Feinstein or anyone raised the specter of new ‘information’ about him.”

Feinstein had been extremely coy about the contents of the letter, refusing to answer questions about the matter before releasing her statement on Thursday.

Kavanaugh’s nomination will receive a final vote in the committee next Thursday. Absent a bombshell revelation, he is likely to advance on a party-line vote, clearing the way for full Senate consideration the following week.

This story has been updated with additional reporting.