Supreme Court Nominee Brett Kavanaugh testifies during the second day of his Supreme Court confirmation hearing on Capitol Hill September 5, 2018 in Washington, DC.

Senators started reviewing the FBI's supplemental background investigation into sexual misconduct allegations against Supreme Court nominee Brett Kavanaugh on Thursday.

President Donald Trump last week ordered the agency to reopen its inquiry into the judge, after a number of undecided senators pressed for further investigation into allegations of sexual misconduct. Since Friday, the FBI has interviewed nine people about the allegations, a source briefed on the investigation told NBC News.

There has been much debate over the scope of the inquiry, given the narrow parameters originally set by the White House. A particular point of contention has been the decision not to include interviews with Christine Blasey Ford, the first woman to come forward with an allegation against the judge. The FBI also did not interview Kavanaugh. The absence of the two from the latest review has raised alarm among some top Democrats.

Nonetheless, the White House is confident that the inquiry will bolster Kavanaugh's odds. Raj Shah, a deputy White House press secretary detailed to the Kavanaugh confirmation process, told CNN on Thursday that after senators review the FBI material, "they're going to be comfortable confirming Judge Kavanaugh."

Senate Majority Leader Mitch McConnell, R-Ky., has vowed to have a vote on Kavanaugh's confirmation this week. A procedural vote on the nomination is expected Friday. The final confirmation vote could come Saturday.

All eyes are on a handful of senators who have not indicated how they will vote. In particular, Republican Sens. Lisa Murkowski of Alaska and Susan Collins of Maine are thought to be crucial. Sen. Jeff Flake, R-Ariz., who led the charge last week to re-open the FBI investigation, will also be closely watched.

The Senate is narrowly divided, with the GOP holding a two-vote majority. If all of the Democrats vote down Kavanaugh's confirmation, two Republicans would still be required to sink him. Vice President Mike Pence would vote in the event of a 50-50 tie.

Trump nominated Kavanaugh in July to replace retiring Justice Anthony Kennedy, who cast numerous swing votes in major rulings.