(CNN) As the FBI races to finish its supplemental background check into Judge Brett Kavanaugh , individuals who believe they have useful information about Kavanaugh's behavior or experience have tried with varying degrees of success to share their information with the FBI.

The efforts have led to a frustration that information some classmates posit as important is not being included in the FBI probe, but may reflect the reality that as a supplemental background check, the FBI's focus may be narrower than they would be in a criminal investigation.

Carrie Cordero, a CNN legal analyst and former counsel to the US assistant attorney general for national security, told CNN that it is also not unusual that someone would contact the FBI with information and then not hear back.

"As a general matter, if the FBI is conducting an investigation," Cordero said. "They decide who they need to talk to."

A series of individuals who attended Yale with Brett Kavanaugh say they have either not been contacted by the FBI as part of their renewed background investigation or have tried to make contact with the FBI as the agency continues its supplemental background check into Kavanaugh.

Trying to make contact

In interviews with several classmates -- none of whom claim they were direct witnesses to the incident in which Deborah Ramirez alleges in The New Yorker that Kavanaugh exposed himself to her during a freshman party -- former Yale students tell CNN they still believe they have some information and either have not been contacted at all or have tried to reach out and have not been contacted back by the FBI.

As of Tuesday evening, Liz Swisher, a former classmate of Kavanaugh's who had appeared on CNN and questioned Kavanaugh's truthfulness about his drinking at Yale in testimony before the Senate Judiciary Committee, told CNN she had not been contacted by the FBI. Chad Ludington, another former classmate who said he often drank with Kavanaugh during their early years at Yale, also filled out a form, but had not been contacted back by the FBI as of Wednesday afternoon. Jamie Roche, Brett Kavanaugh's Yale freshman roommate, has not been contacted by the FBI as of Wednesday morning, his spokesman, Peter Kauffman said.

Kerry Berchem, who graduated Yale in 1988 and knew both Kavanaugh and Deborah Ramirez, said the FBI has not contacted her as of Wednesday even though she sent multiple emails to FBI agent J.C. McDonough beginning Sunday.

Berchem said she has texts messages with a college friend who described communications with Kavanaugh in September regarding The New Yorker article that detailed Ramirez's allegations, according to Berchem's emails.

McDonough advised Berchem to contact her local field office and submit her information to the FBI's tip system, and Berchem responded that she did both.

"I have made no accusations nor have I drawn any conclusions (about) what the texts may or may not mean," Berchem said. "I only requested that the FBI contact me."

Mark Krasberg, an assistant professor of neurosurgery at the University of New Mexico who was also a classmate of Kavanaugh's and Ramirez's without direct knowledge of the alleged incident, had also not heard back from the FBI despite numerous attempts to reach out to lawmakers' offices and FBI offices directly.

According to The New Yorker in September, Krasberg had recounted how "Kavanaugh's college behavior had become a topic of discussion among former Yale students soon after Kavanaugh's nomination" and he tried repeatedly to contact the FBI.

In a lengthy statement to CNN, Krasberg said that he'd reached out to New Mexico Democratic Sen. Martin Heinrich's DC office over the phone as soon as he learned the FBI investigation would be opened up.

He says Heinrich's staff forwarded his information to the Senate Judiciary Committee, which he then said was forwarded to the FBI. Krasberg also said he became increasingly concerned by news reports that the investigation would occur over just one week and felt he needed to try and contact the FBI directly. He called the Colorado office where he said he did talk to someone over the phone after a long wait, but was only able to get through "approximately 10% of the evidence" he had.

"She asked me to summarize the remaining information, and I told her that I could help identify the location of the incident, and that I knew the names of other witnesses who would contemporaneously back up Debbie Ramirez's story," he said.

On Sunday, he said he reached out to staff for Arizona Sen. Jeff Flake, a key swing vote and undecided Republican, as well as Delaware Sen. Chris Coons, a Democrat on the Senate Judiciary Committee and close friend to Flake, to discuss what he knew. They again forwarded his information to the Senate Judiciary Committee, but as of Tuesday night, Krasberg said he had still not heard back from the FBI.

Another fellow Yale alum who lived in the same dorm as Kavanaugh told CNN that he knew Kavanaugh and Ramirez, and also knew exactly where the alleged party occurred. Speaking to CNN on background he explained that, while he lived on the opposite side of the building, there was a common entrance near the suite where the party occurred and he, along with many others, used that entrance. This person said he certainly believed Ramirez's story, and said he knew that Kavanaugh was someone "who partied" with people who drank a lot. He said he had no direct knowledge of the alleged incident.

This alum said that he knew a lot of the people who lived closer to that side of dorm and who likely could have known about it, and who likely knew them both. He said he believed many of them would or should be able to corroborate the incident. He also said he and a number of his friends from the dorm back then were all talking and emailing together about the alleged incident. This alum told CNN on Sunday that he contacted the FBI by phone, first near his home, and then he was forwarded on to agents in in DC. He told the agents on the phone that he had names of people who would likely know about whether alleged party took place and what happened. He said the FBI seemed very interested on phone, and said they would get back to him. But as of Tuesday he had not heard back from anyone at the FBI, and now "fears" the investigation "could be a joke."

Alan Abramson, a lawyer representing a client who was a friend of Ramirez's, said that his client had also not been contacted. According Abramson, Ramirez told his client in the early 90s about an incident that happened during Ramirez's freshman year at Yale.

"I immediately contacted Ms. Ramirez's attorney and gave him this information including my client's 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," Abramson said. "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."

CNN also tried to contact several others involved, including potential witnesses to the alleged incident between Ramirez and Kavanaugh, who have not returned calls or declined to comment to CNN about whether they had been contacted by the FBI.

Democrats have concerns

That fact that several people say they have either not been contacted or have not heard back from the FBI after making initial contact, may indicate a limited scope to of the FBI's supplemental background investigation, a fact that is creating growing concern among Senate Democrats as the timeline for the investigation draws to an end.

"A number of Democratic senators have expressed private concerns about the problem of constituents not getting call backs and not knowing how to direct them to the right place in the FBI," a Democratic senator said.

New York Democratic Sen. Kirsten Gillibrand told CNN's Wolf Blitzer on "The Situation Room" that she passed on a letter to the FBI "of someone who wants to be interviewed, who has relevant information and has asked to be part of this further background check." Gillibrand said she does not know the contents of the letter, or how serious the details may be.

The senator added she has also heard of other witnesses who say they have relevant information who have not been followed up with by the FBI.

Gillibrand told CNN she does not believe the FBI is doing a thorough investigation.

"That doesn't sound like the complete, thorough investigation that frankly senators deserve and would be due if you had a thorough background check," she said.

The FBI has been working very closely with the White House and has been keeping White House counsel Don McGahn updated on their work. The details of the first interviews were shared with the White House and that is how some of the additional interviews then came about.

One of those interviewed by the FBI was Ramirez, who alleged that Kavanaugh exposed himself to her and others in a dorm room when they were Yale freshmen. Ramirez's lawyers provided the FBI with the names of more than 20 individuals who they say can help corroborate Ramirez's story. Only a small number of those, however, may have witnessed the alleged incident. It is those individuals to whom the FBI may also be talking. Those interviews would require the White House's approval.

Ramirez's lawyer John Clune said in a tweet on Tuesday that they are "not aware of the FBI affirmatively reaching out to any of those witnesses."

Why the FBI may have a limited scope

In high-profile investigations, it is not uncommon for the FBI to receive large volumes of tips from the public, which must be independently vetted to determine their credibility and applicability to a particular investigation, says CNN law enforcement analyst Josh Campbell.

With the clock ticking on the Senate's expected vote on Kavanaugh by week's end, it remains to be seen whether the FBI will have sufficient time to conduct a comprehensive review.

The bureau has declined to comment on the record about the ongoing background investigation, however, a US government official familiar with the Kavanaugh review tells CNN the FBI remains in "information collection mode" and is sorting through a number of leads from the public submitted via its online tip portal, toll-free call center and various field offices.

James Gagliano, a retired FBI supervisory special agent and CNN law enforcement analyst, said the FBI examines every single tip it receives, so if individuals have submitted information but have not been contacted, that doesn't mean the FBI has ignored their information.

"The mantra of the FBI is turn over every stone and don't miss anything," Gagliano said. "If they are not returning calls, there is a reason for it."

Republicans in Congress maintain they have confidence in the FBI investigation.

In a statement Tuesday evening, Senate Judiciary Chairman Chuck Grassley spoke out about the FBI's ongoing investigation saying he was "confident that the FBI agents tasked with this responsibility will not succumb to public political pressure or politicians telling the agency how to do its job."

"Respectfully, the career public servants and professionals at the FBI know what they're doing and how best to conduct a background investigation. The FBI's business should be carried out independent of political or partisan considerations. I hope my Democratic colleagues do not attempt to interfere in that process," Grassley said.

It's unclear when the FBI's investigation will be completed although the agreement was that it would be finished within one week when Flake asked for it last week. It's also not certain whether the findings would be released publicly. Traditionally, information obtained through background investigations are not revealed publicly although some lawmakers have argued that the Kavanaugh investigation was not typical and that it could be important to make some pieces of it public.