Texas authorities announced sexual assault charges Friday against disgraced gymnastics doctor Larry Nassar — who is already locked up for the rest of his life — and a trainer he worked with at the ranch owned by legendary coaches Bela and Martha Karolyi.

But prosecutors said there was no evidence of criminal wrongdoing by the Karolyis, and statutes of limitations prevented them from charging USA Gymnastics officials who didn't immediately report suspected abuse to law enforcement.

That angered some Nassar victims who say the Karolyis and USA Gymnastics should be held accountable for failing to protect young athletes — some of them Olympic champions — who were under their supervision.

"Charging Larry Nassar with more crimes makes about as much sense as digging up Lee Harvey Oswald and charging him with JFK’s murder," said attorney John Manly, who represents many of the more than 300 accusers.

Nassar, who has already pleaded guilty to molesting former patients in Michigan, was indicted by a Texas grand jury on six counts of sexual assault. Trainer Debbie Van Horn was charged with one count of sexual assault. Neither could be immediately reached for comment.

The charges are linked to six former patients who were treated at the Karolyi Ranch in Huntsville that was used as a training center by USA Gymnastics. The incidents date to the early 2000s.

Although Walker County District Attorney David Weeks said the investigation was still open, Assistant DA Stephanie Stroud said that at this point the Karolyis are off the hook.

"The Karolyis were and remain fully cooperative and there is no corroborated evidence of criminal conduct by Bela or Martha," she said.

Nassar survivors have been pushing for charges against the Karolyis, saying the couple ignored signs of abuse and fostered a punishing atmosphere that made stressed-out gymnasts vulnerable to grooming by a cunning predator.

In an exclusive interview with NBC News' Dateline in April, the Karolyis said they had no idea that Nassar was abusing girls. Their attorney, David Berg, said they had been "exonerated."

"There is no comparison to what the girls have gone through, but Martha and Bela are also victims of Larry Nassar, and of his assistant Debbie Van Horn. They have devoted their lives to gymnastics and to the girls they have trained. This has just killed them," he said.

"It has been clear to our law firm from the start that the allegations and civil suits against them have been entirely opportunistic. They are seen as a 'deep pocket,' but really should not be involved at all."

Even though USAG and its former executives escaped charges, they were heavily criticized by prosecutors for not going to law enforcement as soon as a report of suspected abuse was made in June 2015.

"It is our belief there was a total failure by USAG to protect the athletes that were part of their program and to take appropriate action once they were made aware of Dr. Nassar's actions," Stroud said.

As NBC News has previously reported, USAG hired a consultant to investigate the initial allegation and waited five weeks before calling the FBI.

"They received information and did not act on it timely," Weeks added.

USAG did not notify Nassar's employer, Michigan State University, that he was under investigation, and he continued to molest patients there for more than a year. USAG has said it was acting on the advice of the FBI, which has ordered an internal review of the slow pace of its own probe.

Nassar was not exposed as a prolific sexual abuser until the fall of 2016. At that point, Texas investigators received several reports of abuse at the ranch but turned over those cases to the feds to be included in Nassar's plea deal.

He ultimately pleaded guilty to possession of child pornography and to molesting 10 girls and was sentenced to up to 175 years in prison. But public outrage spurred the Texas governor to order the Rangers to investigate whether there was reason to charge anyone else for crimes committed in the state.

Asked why the state was bothering to prosecute Nassar when any conviction won't extend his sentence, Weeks said, "Because it's the right thing to do."

Manly, the survivors' lawyer, said he was gratified that Van Horn was charged, but maintainde that's not enough.

"There are at least five adults that were aware Nassar was molesting little girls and failed to report it," he said. "The message to people who were in charge of protecting children is that, in Texas, if you fail to report molesters nothing will happen to you."