Senate officials say they cannot legally release any potential records relating to a complaint purportedly made by a former aide to Joe Biden who has since accused him of sexual misconduct.
Their rebuffal followed a request to see them by the presumptive Democratic presidential nominee.
Tara Reade has accused Mr Biden of sexually assaulting her in 1993 when he was a senator for Delaware, although she has said a formal complaint she filed that year describes only harassment, not assault.
Mr Biden, who denies all the allegations, has responded in part by calling on the Senate to release all the documentation pertaining to the complaint Ms Reade says she filed.
Responding to a letter from Mr Biden, the secretary of the Senate's office, on the advice of legal counsel, concluded the "Secretary has no discretion to disclose any such information as requested".
The office, which did not confirm or deny the complaint's existence, based its decision in part on a review of confidentiality requirements.
The letter from Mr Biden was related to Ms Reade's allegation that he reached up her skirt and penetrated her with his fingers in 1993.
Mr Biden has unequivocally denied the allegations and added he did not recall any complaint being filed.
Ms Reade told the 'Washington Post' her complaint dealt with harassment, not assault. She has said she cannot recall the office to which she made her complaint and that she does not have a copy.
She was employed in Mr Biden's Senate office for nine months, ending in 1993.
She said she filed a complaint that year with a congressional human resources or personnel office.