“We can only conclude that the F.B.I. — or those controlling the investigation — did not want to learn the truth behind Ms. Ramirez’s allegation,” the lawyer, William Pittard, wrote.

Dr. Blasey’s lawyers provided the F.B.I. with the names of eight people they argued should have been interviewed, in addition to their client, and offered to provide additional evidence, including medical records. They said on Thursday that they got no response.

“The ‘investigation’ conducted over the past five days is a stain on the process, on the F.B.I. and on our American ideal of justice,” the lawyers wrote.

An official who reviewed the F.B.I.’s material said the bureau contacted 10 people and interviewed nine of them. The 10th person refused to be interviewed. Those interviewed included Ms. Ramirez and three people Dr. Blasey recalled being in the house at the time of the party: Mr. Judge, P.J. Smyth and Leland Keyser. All three have said they did not remember the party or witness misbehavior by Judge Kavanaugh, although Ms. Keyser told The Washington Post that she believes Dr. Blasey. Republicans briefed on their F.B.I. interviews said those witnesses did not change their accounts.

Also interviewed were two other high school friends of Judge Kavanaugh: Chris Garrett and Tim Gaudette.

But the four undecided senators will determine the fate of Judge Kavanaugh, 53, a 12-year veteran of the federal appeals court in Washington. Mr. Manchin said he would need to return on Friday before the vote to continue reviewing the F.B.I. summaries, but he said Ms. Heitkamp’s decision would not affect his.

The F.B.I. did not publicly explain why it stopped after talking with just nine people. The bureau apparently did not explore allegations by a third accuser, Julie Swetnick, who is represented by Michael Avenatti, a lawyer who also works for Stephanie Clifford, the former pornographic film actress known as Stormy Daniels who was paid hush money before the 2016 presidential election to keep her from discussing what she said was an extramarital affair with Mr. Trump.