Senate Judiciary Chairman Charles E. Grassley does not plan to change a Sept. 20 vote on Brett Kavanaugh because of a mysterious letter about the Supreme Court nominee’s past that was referred to “federal investigative authorities,” a committee spokesman said Thursday.

Sen. Dianne Feinstein of California, the panel’s top Democrat, revealed in a cryptic news release Thursday that she had information about Kavanaugh but was keeping it confidential at the request of the individual who provided the information.

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

Feinstein’s statement did not reveal the nature or seriousness of this matter, or if this is now an ongoing probe. The Federal Bureau of Investigation does background checks for federal judicial nominees.

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Republican Sen. John Cornyn of Texas, a member of the committee, took to Twitter to cast doubt on the issue. “Let me get this straight: this is statement about secret letter regarding a secret matter and an unidentified person. Right,” Cornyn wrote.

News stories about a letter from a California constituent that first went to Democratic Rep. Anna G. Eshoo appeared Wednesday and Thursday morning and might have prompted Feinstein’s statement. The Intercept reported that other Democrats on the committee wanted to see the letter, but Feinstein had refused.

The New York Times reported Thursday that officials familiar with the matter say it involved possible sexual misconduct between Kavanaugh and a woman when they were both in high school.

The letter could feed into committee Democrats’ argument that Republicans are rushing through the Kavanaugh confirmation without a thorough vetting of the nominee, who has been an appeals court judge in Washington for 12 years.

“Senator Grassley is aware of Senator Feinstein’s referral,” Taylor Foy, a committee spokesman, said in an email. “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.”

The White House defended Kavanaugh, saying the FBI has “thoroughly and repeatedly vetted” him throughout 25 years of public service.

“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,” White House spokeswoman Kerri Kupec said in a statement. “Not until the eve of his confirmation has Sen. Feinstein or anyone raised the specter of new ‘information’ about him.”