Olympic gold medallist Simone Biles and dozens of other women who say they were sexually assaulted by Larry Nassar are seeking more than $US1 billion ($A1.39 billion) from the FBI for failing to stop the sports doctor when the agency first received allegations against him, lawyers said on Wednesday.

There's no dispute that FBI agents in 2015 knew that Nassar was accused of assaulting gymnasts, but they failed to act, leaving him free to continue to target young women and girls for more than a year. 

He pleaded guilty in 2017 and is serving decades in prison.

"It is time for the FBI to be held accountable," said Maggie Nichols, a national champion gymnast in Oklahoma in 2017-19.

Under US federal law, a government agency has six months to respond to the tort claims filed on Wednesday.

Lawsuits could follow, depending on the FBI's response.

The approximately 90 claimants include Biles, Aly Raisman and McKayla Maroney, all Olympic gold medallists, according to Manly, Stewart & Finaldi, a California law firm. Separately, 13 claims were filed by others in April.

Agents heard Nassar allegations in 2015

Indianapolis-based USA Gymnastics told local agents in 2015 that three gymnasts said they were assaulted by Nassar, a team doctor.

But the FBI did not open a formal investigation or inform federal or state authorities in Michigan, according to the Justice Department's inspector general, an internal watchdog.

Los Angeles agents in 2016 began a sexual tourism investigation against Nassar and interviewed several victims but also didn't alert Michigan authorities, the inspector general said.

Nassar wasn't arrested until the fall of 2016 during an investigation by police at Michigan State University, where he was a doctor.

The Michigan attorney general's office ultimately handled the assault charges against Nassar, while federal prosecutors in Grand Rapids, Michigan, filed a child pornography case.

Failure to 'stop this monster… inexcusable'

The FBI declined to comment Wednesday on the former gymnasts' financial claims, referring instead to remarks last year by FBI Director Christopher Wray, who acknowledged major mistakes.

At that same hearing, Biles, widely considered to be the greatest gymnast of all time, said an "entire system" enabled the abuse.

Maroney recalled "dead silence" when she talked to FBI agents about Nassar.

The Justice Department in May said that it would not pursue criminal charges against former agents who were accused of giving inaccurate or incomplete responses during the inspector general's investigation.

Failures by federal law enforcers have led to major settlements, including $US127.5 million for families of those killed or injured in 2018 at Florida's Marjory Stoneman Douglas High School.

The FBI received a tip about five weeks before 17 people were killed, but the tip was never forwarded to the South Florida office.

Michigan State University, which was also accused of missing chances over many years to stop Nassar, agreed to pay $US500 million to more than 300 women and girls who were assaulted by him.

USA Gymnastics and the US Olympic and Paralympic Committee made a $US380 million settlement.

AP