"It is not the practice of the FBI to decline to pursue credible leads in an investigation or to fail to interview the accuser and accused," the two senators wrote.

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The letter comes after Wray testified late last month before the Judiciary Committee that the FBI's background investigation into the sexual misconduct allegations against Kavanaugh, who has denied the claims, was in line with bureau procedures.

"I consulted at length with our security professionals who are specialists in background investigations to make sure that investigation, that background investigation, was done consistent with our long-standing policies, practices, and procedures for background investigations," Wray said during the hearing.

But Coons and Whitehouse raised concerns in their letter that the White House and Senate Republicans, or their staffs, were involved in setting the boundaries of the FBI's probe.

"In addition, the process used to begin and conduct the investigation does not appear to meet even the minimal steps taken by the FBI when allegations were raised against Justice [Clarence] Thomas," the two senators added in the letter to Wray.

The two senators are asking Wray to detail the FBI's procedures for handling allegations of sexual misconduct that come up during a background investigation of a nominee, how they are different from criminal investigations or background investigations that don't involve sexual assault allegations and what procedures were used during the Kavanaugh investigation.

They also want to know what limits were placed on the FBI's investigation by the White House or Justice Department, who identified which witnesses the FBI would interview, how many people were interviewed and the process for questioning and following up on interviews.

The two Democratic senators are also raising concerns about a public tip line established during the FBI's investigation. They want to know if the FBI has previously used a tip line, how many tips were received and how they were vetted.

Reports surfaced around the FBI investigation that the bureau had not spoken to dozens of potential witnesses and that individuals with possible information had not heard back from the FBI.

Republicans agreed to delay a final vote on Kavanaugh's Supreme Court nomination last year to give the FBI one week to do a supplemental background investigation into sexual assault allegations raised against him, including Christine Blasey Ford's accusation that Kavanaugh pinned her to a bed and tried to remove her clothes at a house gathering while the pair where in high school in 1982. As part of the supplemental investigation, the FBI didn't interview Ford or Kavanaugh.