Two sources briefed on the matter said it relates to allegations of misconduct by Kavanaugh while he was in high school. There is no criminal investigation into Kavanaugh as a result of this letter, an FBI official said.
"I have received information from an individual concerning the nomination of Brett Kavanaugh to the Supreme Court," Sen. Dianne Feinstein, the top Democrat on the Senate Judiciary Committee, 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," she added.
Sources familiar with the matter say the letter Feinstein referred to the FBI had all the names redacted except for Kavanaugh's.
Additionally, the sources say the information wasn't disclosed elsewhere and the reason Feinstein didn't come forward earlier with the information because the woman didn't want to speak about it and didn't want her name public.
A source familiar with the allegations says they stem from a sexual encounter that occurred when Kavanaugh was about 17 years old. Another source familiar with the matter says the information concerns allegations of misconduct against Kavanaugh. Sources caution, however, that few have seen the actual letter Feinstein referred to the FBI. A Democratic source says California Rep. Anna Eshoo had referred a letter about Kavanaugh from one of her constituents to Feinstein.
Multiple sources also said Feinstein received the letter in July, well before she met with the nominee and prior to Kavanaugh's confirmation hearings last week. It is not clear why the information was not relayed to the FBI until Wednesday night.
A source familiar with the process says there is no criminal investigation into Kavanaugh as a result of this letter. It is part of Kavanaugh's background file and it will be up to the White House to determine whether there is any further action to take. The Judiciary Committee received an updated supplement to Kavanaugh's background investigation from the FBI Thursday afternoon, an aide said. Senators on the committee can now request access to that information.
Patrick Coyle, a spokesman for Georgetown Preparatory School, the private prep school in Maryland that Kavanaugh attended before college, responded to inquiries about accusations against Kavanaugh: "We have no knowledge regarding any accusation."
The White House, in a statement shortly after Feinstein's, said Kavanaugh has been "thoroughly and repeatedly vetted" by the FBI for government positions over the past 25 years and accused Democrats of trying to delay the nomination.
"Throughout his confirmation process, Judge Kavanaugh has had 65 meetings with senators -- including with Sen. Feinstein -- sat through over 30 hours of testimony, addressed over 2,000 questions in a public setting and additional questions in a confidential session. Not until the eve of his confirmation has Sen. Feinstein or anyone raised the specter of new 'information' about him," said White House spokeswoman Kerri Kupec.
The Judiciary Committee is scheduled to vote on Kavanaugh's nomination next Thursday, with the full Republican-led Senate looking to consider it later this month.
Democratic Sen. Dick Durbin called for a delay, saying the Judiciary panel should not vote on Kavanaugh "until all the information is before the committee."
"No idea what they are going to find," Durbin told CNN. "I think the FBI is the appropriate agency and that's where it was referred."
The allegations, in the form of a secret letter, add a new public element to what has been a divisive and highly partisan nomination process for Kavanaugh.
But it's an issue that has been bubbling beneath the surface on Capitol Hill for days -- with rumors circulating amongst staffers about the existence of a confidential letter, aides said.
The issue came to a head Wednesday night, when Democrats on the Judiciary Committee met and discussed the letter for the first time. Feinstein described the allegations in the letter but declined to make it available to colleagues, one source briefed on the matter said. She chose instead to refer it to the FBI, which conducts background checks on federal nominees.
The FBI confirmed it had received the referral.
"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 said.
Republicans on both Capitol Hill and inside the White House decried the way the letter and allegations had become public.
"It smacks of desperation to me," said Sen. John Cornyn, R-Texas.
Judiciary Chairman Chuck Grassley, R-Iowa, said he has no plans to delay the vote.
"Sen. Grassley is aware of Sen. Feinstein's referral," Grassley spokesman George Hartmann said. "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."
And the White House accused Senate Minority Leader Chuck Schumer of trying to delay the nomination.
"Sen. 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.
Yet Schumer, along with other Democratic senators, still had not seen the actual letter itself, multiple aides confirmed.
"Sen. Schumer has not had access to the letter but believes the Judiciary Committee is handling it appropriately," a Schumer aide said.
CNN's MJ Lee, Jessica Schneider, Ted Barrett and Manu Raju contributed to this report.