A senate committee earlier voted to approve Brett Kavanaugh's nomination to the top us court.
But a republican member Jeff Flake only backed the move on the understanding that such an inquiry would occur.
As a result, a full senate vote on Judge Kavanaugh's confirmation has been delayed for up to a week.
Judge Kavanaugh, an appeal court judge, denies allegations from at least three women.
On Thursday, the committee heard testimony from Christine Blasey ford, a professor of psychology in California, who says judge Kavanaugh sexually assaulted her when they were teenagers in the 1980s.
Judge Kavanaugh also testified, angrily rejecting the allegation he had ever assaulted her or anyone else.
He accused democrats of politicizing the process and harming his family and good name.
Despite ordering the FBI investigation, president trump has continued to Defend Judge Kavanaugh.