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WASHINGTON — The FBI's much-awaited supplemental background investigation into Supreme Court nominee Brett Kavanaugh was delivered overnight to Capitol Hill and lawmakers started reading it in a secure area on Thursday morning.

It consists of the "302" forms of the FBI interviews, which summarize the contents of the interviews, according to aides and senators. The FBI, which has spent only a few days on the investigation, did not submit a conclusion as to who's telling the truth in the case.

All 100 Senators will have access to the new information, but not their staffs. There also are 10 Judiciary Committee staffers who do have access to the secret Kavanaugh file, which is a paper report — there are no PDFs or emails of it. And it will not be made public.

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Majority Leader Mitch McConnell, R-Ky., set a key procedural vote for Friday that would set the stage for a possible full Senate vote as early as Saturday.

On Thursday, Senate Judiciary Committee Chairman Sen. Chuck Grassley, R-Iowa, was the first to be briefed by staff on the Kavanaugh file at 8 a.m. and then ranking Democratic member Sen. Dianne Feinstein, D-Calif., can read the file at 9 a.m., sources briefed on the schedule told NBC News. Following that, the rest of the GOP members on the Judiciary Committee go at 10 a.m. and the Democrats on the panel can view the report at 11 a.m.

Others senators will have access to the report afterward. Allowing all 100 senators to view the report if they want to in a timely fashion is a priority because of McConnell's quick timeline for a vote of the full Senate.

There are not multiple copies of the background investigation file, and senators cannot go pick it up and bring it home with them.

Senators are viewing the file in the Senate SCIF (Sensitive Compartmented Information Facility), which is the classified area of the Capitol Visitor's Center.

According to committee aides and a document dictating how the file is to be handled, "The Security Manager shall maintain in a locked safe a log that reflects the date, time, and particular FBI background investigation report received by the Committee."

The information in the background investigation file is not marked top secret or classified, but it is not to be leaked or even characterized. Senators are "not allowed to share any details whatsoever," a committee aide said.

That rule will likely be tested.