Several US senators have called for the FBI agents who botched the investigation into sexual assault allegations against the former USA Gymnastics doctor Larry Nassar to be prosecuted during a congressional hearing this morning.

"I understand it's a long-standing department policy not to comment on decisions, not to prosecute, but robust oversight of the Department of Justice is a core responsibility of this committee,” Senate Judiciary Chairman Dick Durbin said in his opening remarks. “The FBI is handling of a Nassar's case is a stain on the bureau.”

Four members of the Olympics USA Gymnastics team are testifying before the committee. The hearing is being held after scathing report from the Justice Department's inspector general's office this summer revealed a number of missteps by the FBI in the case.

Nassar is currently serving a 40-to-174-year state prison sentence after 150 women and girls came forward to expose that he abused them over the course of 20 years.

"Today we believe Nasser abuse more than 300 athletes before he was brought to justice. As the details of Nasser's crimes emerged, there's been a consistent theme of neglect and inaction by those who are responsible for protecting the athletes," Durbin said. 

Decorated gymnasts Simone Biles, McKayla Maroney, Maggie Nichols and Aly Raisman as well as the FBI Director Christopher Wray and Inspector General Michael Horowitz are scheduled to testify. 

The inspector general's report found that FBI Special Agent in Charge W. Jay Abbott and Supervisory Special Agent Michael Langeman conducted a "limited follow up" investigation in 2015 and neglected to interview two of the three accusers. 

"I hope to hear more about exactly what happened at FBI Headquarters, other than its children you in it who knew about the Nasser allegations, how and when they learned of these allegations and what they did in response," Sen. Chuck Grassley said in his opening remarks. "If there's one thing the inspector general's report illustrates it says that we need to make sure the bureau is more effective and held more accountable."

Following the inspector general's report, the Justice Department declined to prosecute Abbott and Langeman. Abbott retired and Langeman was fired in recent weeks. 

"It's not only that the FBI failed to do its job, systematically, and repeatedly, it is also the cover of the cover up that occurred afterward,” Sen. Richard Blumenthal said in his opening remarks.

“When FBI agents made material, false statements and deceptive omissions referred by the Inspector General for criminal prosecution, those referrals were declined, without explanation, without any public explanation at all," Blumenthal said. "My hope is that the Department of Justice, which was invited today and has declined to appear, will explain why those lies by FBI agents did not lead to criminal prosecution, and accountability and even days before this hearing."

Durbin said he's "disappointed" that the Justice Department declined to participate in Wednesday's hearing but promises an oversight hearing with them in the fall.

Grassley said that he is working on "legislation to close the legislative loophole in the sex tourism statute that the inspector general flag in his report. This gap in the law allowed Larry Nasser to evade federal prosecution for assaulting children while traveling abroad, and that can never happen again."