Senate Democrats have referred a letter concerning Supreme Court nominee Brett Kavanaugh to the FBI for investigation.

"I have received information from an individual concerning the nomination of Brett Kavanaugh to the Supreme Court. That individual strongly requested confidentiality, declined to come forward or press the matter further," Sen. Dianne Feinstein (D-Calif.) said on Thursday.

"I have honored that decision. I have, however, referred the matter to federal investigative authorities," she continued.

A source familiar with the matter confirmed to The Hill that the letter had been referred to the FBI.

Democrats on Thursday began to publicly acknowledge the existence of the letter that had previously been rumored on Capitol Hill.

Sen. Dick Durbin (Ill.), the No. 2 Senate Democrat and a member the Judiciary Committee, told BuzzFeed News that it had "been referred to the FBI for investigation."

But lawmakers are tightlipped about the contents of the letter and have not publicly disclosed what it says or what it is about, beyond being broadly related to Kavanaugh.

Over the past two days, Feinstein has declined to answer questions from reporters on Capitol Hill about the letter, which was reportedly first given to Calif. Rep. Anna Eshoo (D).

Questions about the letter boiled over on Wednesday after The Intercept published a report saying Feinstein had refused to share the content of the letter with other members on the committee.

Sources told The Intercept that the letter was relayed to lawmakers by an individual affiliated with Stanford University. Some sources told the publication that it was about an incident involving Kavanaugh and a woman while they were in high school.

The New York Times separately reported, citing two sources familiar with the matter, that it the incident involved possible sexual misconduct.

Attorney Debra Katz is believed to be representing the woman who is the subject of the letter, though she did not confirm that to The Intercept. She also declined to confirm to BuzzFeed News that she is representing the woman, and did not comment further.

"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," an FBI spokesperson told The Hill.

A government official confirmed that a criminal investigation has not been opened into the matter.

The letter comes as Kavanaugh's nomination is moving forward in the Senate despite an intense fight over documents and questions from Democrats about if he misled the Judiciary Committee.

Sen. Chuck Grassley (R-Iowa), the chairman of the committee, has scheduled a panel vote on his nomination for Sept. 20 at 1:45 p.m. Because Republicans hold a majority on the panel his nomination is expected to be sent to the full Senate late next week.

George Hartmann, a spokesman for Grassley, said that the GOP senator had not seen the letter, but there were no plans to change the committee's consideration of Kavanaugh.

"Senator Grassley is aware of Senator Feinstein's referral. At this time, he has not seen the letter in question, and is respecting the request for confidentiality. There's no plan to change the committee's consideration of Judge Kavanaugh's nomination," he said.

A Judiciary Committee spokesperson confirmed that they had received an update to Kavanaugh's background file on Thursday.

Several moderate senators in both parties remain undecided and their votes on Kavanaugh will make or break whether he gets confirmed later this month.

GOP Sen. Susan Collins (Maine), one of the potential swing votes, told reporters that she had not seen the letter. She is expected to speak with Kavanaugh by phone on Friday - a meeting that was scheduled before reports of the letter surfaced.

Brian Fallon, a spokesman for Demand Justice, which opposes Kavanaugh's nomination, said in a tweet that "undecided senators should be asked if they still see 'no red flags' on Brett Kavanaugh."

Republican lawmakers and the White House immediately defended Kavanaugh.

Sen. John Cornyn (Texas), a member of the Judiciary Committee, took to Twitter to cast doubt on the letter.

"Let me get this straight: this is statement about secret letter regarding a secret matter and an unidentified person. Right," the No. 2 Senate Republican said in a tweet.

He added that the FBI already conducted a background investigation on Kavanaugh, which is available for senators to review.

The White House dismissed the letter as a last-ditch attempt by Democrats to derail Kavanaugh's nomination.

"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," White House spokesperson Kerri Kupec said in a statement.

But Minority Leader Charles Schumer's (D-N.Y.) office said on Thursday that the Senate Democratic leader "has not had access to the letter but believes the Judiciary Committee is handling it appropriately."

-Jordan Fabian contributed to this report, which was last updated at 6:58 p.m.