WASHINGTON — The organizations that should have protected young female gymnasts from sexual abuse by former Olympic gymnastics team doctor Larry Nassar, including the U.S. Olympic Committee and the FBI, "fundamentally failed" to do so for years, according to a new congressional report.

In an interview with NBC News, Sen. Richard Blumenthal, D-Conn., the ranking member of the Senate subcommittee overseeing the Olympics, described the actions of the organizations as a "cover-up."

"Whether it was a criminal cover-up remains to be proven, but it was a cover-up in spirit," he said.

The scathing report, obtained by NBC News before publication, outlines the findings of an 18-month investigation by Blumenthal and the other members of the Senate Commerce Subcommittee on Manufacturing, Trade, and Consumer Protection. It is accompanied by proposed legislation to improve oversight and prevent future abuse that will be introduced Tuesday.

"Terrible things happened," Sen. Jerry Moran, R-Kan., the chair of the subcommittee, told NBC News. "In many instances they were reported and, almost without exception, the people that they were reported to didn't respond."

Moran and Blumenthal, who sat down for an interview with NBC News together, had previously referred Scott Blackmun, the former CEO of the U.S. Olympic Committee, to the Justice Department for possible criminal prosecution for making false statements to Congress.

The report concluded that Michigan State University, where Nassar also worked, USA Gymnastics, and the U.S. Olympic Committee, as well as the FBI, "had opportunities to stop Nassar but failed to do so."

The report details how Nassar was able to abuse more than 300 athletes over two decades because of ineffective oversight by the Olympic organizations. The findings confirm previous reporting by NBC News, and cite an NBC News interview with McKayla Maroney that aired on "Dateline" in April 2018.

Between summer 2015 and September 2016, the Olympic organizations "knowingly concealed abuse by Nassar, leading to the abuse of dozens of additional amateur athletes," according to the report. The Olympic Committee, USA Gymnastics and MSU all received reports about Nassar's abuse more than a year before anything was done, the investigation found.

"The Olympic-related organizations' ability to identify and prevent abuse was inadequate," the report said. "As a result, hundreds of women and girls were sexually abused by Larry Nassar."

Victims and supporters look on as Rachael Denhollander speaks at the sentencing hearing for Larry Nassar in Lansing, Michigan, on Jan. 24, 2018. Brendan McDermid / Reuters file

USA Gymnastics told NBC News in April 2018 that "USA Gymnastics denies any allegation that it had wide-ranging knowledge of abuse by Nassar or that it concealed or ignored his abuse."

Li Li Leung, president and CEO of USA Gymnastics, said in a statement Tuesday that the organization had not yet seen the report or the proposed legislation, but it had "already made numerous changes designed to prevent the opportunity for abuse to occur. We have made it our top priority to become an athlete-centric organization that keeps athlete safety and well-being at the forefront of all that we do."

Leung said USA Gymnastics will implement "most of the recommendations made in an independent, investigative review of our safe sport policies and procedures."

"We admire the survivors' courage and strength in sharing their stories, and our goal is to do everything we can to prevent the opportunity for it to happen again."

In an open letter to athletes in December following its own investigation, the U.S. Olympic Committee acknowledged institutional failures and said it was implementing reforms.

U.S. Olympic Committee CEO Sarah Hirshland told NBC News on Tuesday that "increasing accountability" is central to the organization’s ongoing reform.

She also applauded Congress for the proposed legislation and said it is consistent with the Olympic Committee’s approach, but said there were sections in it "that, while conceptually appropriate, could result in unintended consequences and disruption for athletes in operational reality. We look forward to working with Sens. Moran, Blumenthal and others in Congress to address these areas, make athletes more safe, and make Olympic and Paralympic organizations in the U.S. as exceptional as the athletes they serve."

A spokesperson for MSU said the school was unaware of Nassar's abuse until it was reported in the Indianapolis Star in September 2016, and then fired Nassar within days. "We know we let our community and the survivors down by failing to protect them for many years," the spokesperson said. "Over the past few years, the university has invested time, financial resources and staff resources to make improvements in our patient care, sexual assault and relationship violence prevention and also responses to assault."

The FBI opened an investigation into Nassar in July 2015 after receiving credible reports of his alleged sexual abuse of three national team gymnasts — McKayla Maroney, Ally Raisman and Maggie Nichols. But the bureau failed to act and "did not stop Nassar from seeing patients or protect those in harm's way" as the investigation "dragged on" for more than a year as it was shuffled between different field offices, the congressional report found.

Blumenthal and Moran told NBC News that they were not satisfied with the answers the FBI gave them during the investigation. They are still waiting for the results of a Department of Justice Inspector General report on what went wrong at the FBI.

Former Michigan State University and USA Gymnastics doctor Larry Nassar, from center, with defense attorneys Matt Newberg, from left, and Molly Blythe, from right, during the sentencing phase in Ingham County Circuit Court in Lansing, Michigan on Jan. 24, 2018. Jeff Kowalsky / AFP - Getty Images file

"I would say, so far, we've received no satisfactory answer" to the subcommittee's repeated requests as to what the bureau was doing during the more than 400-day gap between when the FBI first learned of the abuse allegations and when Nassar was arrested by local law enforcement, Blumenthal told NBC News. "I think we ought to be demanding, so should the American people, better answers."

The FBI declined to comment to NBC News and referred questions to the Department of Justice Office of the Inspector General. The OIG's office told NBC News it does not comment on ongoing investigations.

The senators are introducing the bill Tuesday, the Empowering Olympic and Amateur Athletes Act, that would increase oversight and improve protections for athletes from sexual abuse.

"The goal here is to change the culture and to do that we needed to change the law," Moran said.

Nassar, who pleaded guilty to sexually abusing 10 minors in Michigan court in January 2018, is serving a prison sentence of up to 175 years and is expected to be behind bars for the rest of his life.

The fallout of the scandal continues. Steve Penny, the former head of USA Gymnastics who resigned amid the scandal, was arrested in October on federal charges of tampering with evidence in connection with a Texas investigation into Nassar.

Penny has pleaded not guilty. His lawyer said in a statement that she could not comment on the congressional report because she had not seen it, but that findings in the report described to her "are simply not true." She said USA Gymnastics had acted with the advice of its legal counsel after receiving a question regarding Nassar's treatment.

Larry Nassar listens during his sentencing at Eaton County Circuit Court in Charlotte, Michigan, Monday, Feb. 5, 2018. Cory Morse / AP file

Facing decertification as a sports governing body and 100 lawsuits representing more than 350 Nassar victims around the country, USA Gymnastics filed for bankruptcy in December 2018. Multiple sources told NBC News at the time that by doing so, the organization was able to put all litigation on hold, including ongoing discovery and depositions of key figures that might have revealed more about who knew of Nassar's conduct and when.

The senators told NBC News that they expect that the findings of their investigation may contribute to additional developments in the near future. "There definitely is fertile ground for additional criminal investigation here and as a former United States attorney, federal prosecutor, I hope they will be undertaken vigorously," Blumenthal told NBC News. "There's a need for accountability here, not just to Congress but to criminal authorities."

"Larry Nassar … was far from a lone wolf," Blumenthal said. "He was enabled by others and if they lied about it and if they obstructed the investigation, if they destroyed documents then they should be held accountable."

Current and former athletes told NBC News that they welcomed the findings of the congressional report and proposed legislation. "There's some amazing steps in this bill that will change the lives and safety of athletes," said Jessica Howard, a former rhythmic gymnastics national champion who says she too was abused by Nassar. "To actually see it get to this point already feels like a huge victory."

Adiel Kaplan and Kenzi Abou-Sabe reported from New York.