FYI:
The Democrats, and the accuser's lawyer, are trying to get the FBI to delay the confirmation by opening an investigation into the allegations. Delay is the objective.
Without rehashing jurisdiction details, the FBI is not supposed to investigate supreme court nominees unless they have jurisdiction. Even if the Senate asks and begs and threatens, the FBI is not going to investigate.
The Senate, and the Senate alone, has the duty and right to "Advice and consent" in supreme court confirmations. The Senate has the duty to investigate if they feel this matter rises to the level of significance in advising and consenting on the nomination.
The Senate has all the resources they need to do this investigation. They could have done this all during the 2 months Feinstein sat on the letter without mentioning it to the FBI or Senate Judiciary Committee -- up to the point where she leaked the letter to the press after the hearings were over.
The call for the FBI to investigate is a red herring. The Senate, not the FBI, has that responsibility.