In 1991, when Hill’s allegations were made public, the FBI got involved at the direction of the White House.

“On Sept. 23, the allegation was brought to the attention of the Judiciary Committee,” a White House statement at the time read. “The Judiciary Committee immediately informed the White House. In consultation with the committee, the White House promptly directed the FBI to conduct a full, thorough and expeditious investigation. Upon completion of the FBI investigation on Sept. 26, the report was submitted to the White House and the committee.”

Note that the investigation only lasted three days, as CNN’s Kevin Liptak points out.

...

Again: This is not unprecedented. The three-day investigation into Thomas may not have fully explored what Hill alleges took place, but it nonetheless occurred, with the White House’s blessing.