WASHINGTON — More than 40 people with potential information into the sexual misconduct allegations against Supreme Court nominee Brett Kavanaugh have not been contacted by the FBI, according to multiple sources that include friends of both the nominee and his accusers.

The bureau is expected to wrap up its expanded background investigation as early as Wednesday into two allegations against Kavanaugh — one from Christine Blasey Ford and the other from Deborah Ramirez.

But sources close to the investigation, as well as a number of people who know those involved, say the FBI has not contacted dozens of potential corroborators or character witnesses.

More than 20 individuals who know either Kavanaugh or Ramirez, who has accused the nominee of exposing himself to her while the two attended Yale University, have not heard from the FBI despite attempts to contact investigators, including Kavanaugh’s roommate at the time and a former close Ramirez friend.

A senior U.S. official and two other sources briefed on the details of the FBI investigation confirmed to NBC news that the FBI’s work on the Brett Kavanaugh matter remains significantly limited in scope, and that it’s unlikely agents will be allowed to interview many, if any, additional witnesses before the probe wraps up this week.

One current and two former FBI officials confirmed to NBC News that dozens of witnesses have come forward to FBI field offices who say they have information on Brett Kavanaugh, but agents have not been permitted to talk to many of them. To the extent that any interviews have been done, the officials say, it’s not clear the information will be considered as part of the FBI’s limited scope inquiry.

Internally, the bureau is concerned that the constraints of the investigation could damage its reputation for finding the truth, the officials said.

Ramirez’s attorney, John Clune, tweeted Tuesday that the FBI "is not conducting — or not being permitted to conduct — a serious investigation." Clune added, "We are not aware of the FBI affirmatively reaching out to any of those witnesses."

Debbie Ramirez spoke to the FBI for over two hours this past Sunday. It was a detailed and productive interview, and the agents were clearly motivated to investigate the matter in any way they were permitted. Ms. Ramirez identified a number of witnesses, and, 1/ — John Clune (@CluneEsq) October 2, 2018

at the end of the interview, her lawyers provided the FBI the names and known contact information of additional witnesses (totaling more than 20) who may have corroborating information. Although we do not know the status of the investigation, 2/ — John Clune (@CluneEsq) October 2, 2018

we are not aware of the FBI affirmatively reaching out to any of those witnesses. Though we appreciated the agents who responded on Sunday, we have great concern that the FBI is not conducting—or not being permitted to conduct—a serious investigation. 3/ — John Clune (@CluneEsq) October 2, 2018

Attorney Alan Abramson told NBC News that he is representing a client who was a friend of Ramirez. "According to my client, in the early 90's, Ms. Ramirez told my client about an incident that happened during Ms. Rameriz’ freshman year at Yale," Abramson said in a statement. "I immediately contacted Ms. Ramirez’ attorney and gave him this information including my clients name. He advised me that he gave my client’s name to the FBI on Sunday, as someone with pertinent information who was willing to speak the FBI. Having not heard from the FBI, I personally contacted them and spoke to two agents. I have not heard from the FBI yet, but I am hopeful that they will still contact me."

"My client has asked me to not disclose any identifying information to the public although the client is willing to go on the record with the FBI," Abramson added. "The FBI has my clients name and my name and contact information."

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The supplemental background investigation was reopened in the wake of last week's Senate Judiciary Committee hearings over Ford's accusation.

The White House has said this week that it has authorized the FBI to expand its initially limited investigation into sexual misconduct allegations against Kavanaugh by interviewing anyone it deems necessary as long as the review is finished by the end of the week, NBC News has reported.

Meanwhile, nearly 20 people with potential information about Dr. Ford’s allegation that Kavanaugh sexually assaulted her while the two were in high school, including Ford herself, have not been interviewed by federal investigators.

Attorneys for Ford said Tuesday that she had not been interviewed by the FBI. She received "no response" after reaching out again to the bureau, according to a letter from her attorneys.

Her team also submitted to the FBI a list of nearly 20 people to turn to, including the polygraph examiner who interviewed Ford, her therapist and friends she confided in about the allegation. According to people familiar with the names on the list, none of them have yet been interviewed by the FBI.

Julie Swetnick, who has accused Kavanaugh of being at parties where boys took turns having sex with inebriated girls in high school, has not been interviewed by the FBI, according to her attorney, Michael Avenatti.

Kavanaugh has strongly denied all three accusations.

The FBI has interviewed at least three additional potential witnesses, most notably Kavanaugh's high school friend, Mark Judge, who Ford said was in the room at the time of the alleged assault. Judge submitted a statement to the Senate Judiciary Committee saying "I do not recall the events described by Dr. Ford in her public testimony."

And Tim Gaudette, a high school classmate of Kavanaugh’s, has been interviewed by the FBI, NBC News has confirmed. The Washington Post reported Tuesday that another classmate, Chris Garrett, was also interviewed.

Ramirez appears to be the only accuser who was interviewed as part of the current investigation.

But several people who know Kavanaugh from his time at Yale told NBC News that they have reached out to the FBI multiple times to provide either corroborating or circumstantial evidence into Ramirez’ allegations that Kavanaugh exposed himself to her at a gathering in a dorm room in 1983. The FBI has yet to respond to their outreach, they say.

Additionally, these former classmates say the FBI would not be doing a thorough investigation if they don’t talk to Kavanaugh’s closest friends during that time, including Kevin Genda and David White.

White lived in the same dorm suite as Kavanaugh and sources say all three hung out often and partied together. Neither Genda nor White have returned multiple requests for comment. NBC News does not know if they have been contacted yet for an interview.

Genda, who works at Blue Torch Feeder Fund in New York City, married Karen Yarsavage, who was also friends with Kavanaugh and Ramirez at Yale. Both Kavanaugh and Ramirez were in their wedding party in 1997.

Another mutual friend, Kerry Berchem, has sent a series of text messages she exchanged with Yarsavage to the FBI.

The texts suggest that Kavanaugh had recently been in contact with Genda and Yarsavage, including in the days leading up to the release of the New Yorker article detailing Ramirez’ allegations.

Despite three attempts to contact the FBI, the bureau has not responded to Berchem, she tells NBC News. Yarsavage has not responded to NBC’s inquiry to determine whether she has been contacted by investigators.

White is another person former classmates say should be a person of interest to the FBI because they say it is plausible he is the person who was referred to in the New Yorker article who ran down the hallway and yelled that Kavanaugh put his genitals in Ramirez’ face.

Another person that multiple sources say the FBI should speak to is Tracy Harmon. She was a close friend of Ramirez’ and they often attended social gatherings together, including those held in the dorm suite. According to sources familiar with the matter, Harmon had not been contacted as of late Tuesday night.

Richard Oh, who lived in the suite where the alleged incident took place, told NBC News that he has tried to contact the FBI twice, once online and once in person.

When he called the first time on Saturday morning, Oh said he struggled to get through. He submitted his information online and reached a person at the bureau over the phone on Sunday morning, he said. He was quoted in the New Yorker as hearing a female student recount the incident to another female student. He said he didn’t know Ramirez but the account of what he heard that night matches Ramirez’ story. Oh is now an emergency room doctor in Northern California.

Another former Yale classmate, Mark Krasberg, has also tried multiple times to reach the FBI but has received no response. He said he wanted to inform them "that I have evidence which would be helpful to the investigation."

Krasberg, who lived in a dorm suite that Kavanaugh often hung out in, said he tried to reach the FBI multiple times, including contacting the Denver office, which is leading the investigation. He was transferred to the national FBI hotline where he spent 45 minutes retelling some of his story.

He has since been in touch with the offices of Sens. Jeff Flake, R-Ariz., and Chris Coons, D-Del., who helped him to get in touch with the FBI. While the FBI hadn’t contacted him as of Tuesday morning, he was told Republican staff on the Judiciary Committee would contact him. He was still waiting for a call as of Tuesday night.

Krasberg, a neurology research assistant professor at the University of New Mexico, acknowledged that he doesn’t have corroborating evidence of Ramirez’ account but that his information might help to connect the dots.

With time ticking on the investigation, Senate Majority Leader Mitch McConnell, R-Ky., reiterated on Tuesday that he plans to hold a final vote on Kavanaugh this week.