A former tennis coach pleaded guilty Tuesday to producing and distributing online videos of himself sexually abusing a young girl to create what federal prosecutors said was one of the most in-demand child pornography series in the world.

James "Bart" Huskey was accused of sexually assaulting a minor and posting progressively violent photos and videos of him and the girl — which authorities have dubbed the "Tara" series — on heavily guarded Internet sites.

The 38-year-old could face up to 70 years in prison and a $750,000 fine after pleading guilty to producing, receiving and distributing child pornography. Local authorities can still charge Huskey with rape and sexual assault charges in the case, but have yet to decide whether to do so.

The guilty plea marked the end of a yearslong investigation that began in Australia and ultimately helped crack what authorities say was a sophisticated child pornography ring that protected members using encrypted Web sites and timed tests meant to verify users' knowledge of child pornography.

Federal authorities began investigating the ring after Australian police alerted them to an investigation in child pornography traders. The FBI agents soon found disturbing videos of a girl as young as six years old being sexually assaulted.

The man in the postings grew more aggressive over time, and the last one, dated November 2007, showed the man holding a large knife toward the girl's body. But authorities struggled to identify the man because his face was often blurred and he sometimes wore a Mardi Gras-style mask.

The group also zealously guarded its members, said federal prosecutor Francey Hakes. It was a "sophisticated and extremely organized" group that required prospective members to pass a timed test and banned them from chatting and phoning other members so they couldn't identify each other in case one was caught, she said.

Hakes said the group also had its own bank account and appeared to have ordered child pornography from outside producers, distributing the material as far away as Canada, England and Germany.

The investigation turned when authorities were able to track down Huskey's location by identifying footage shot from the interior of his car, said Hakes. Prosecutors say footage from other scenes, including scenes shot in a hotel, also helped them locate him.

Federal authorities said there is pending prosecution against others in the ring, but declined to give details.

Huskey stood silently, eyes downcast, throughout the plea. Two of his relatives accompanied him to the courtroom. Both refused comment. Huskey is scheduled to be sentenced Feb. 5.

Prosecutors said the girl, who is now 9, is safe and that she has been referred to programs that provide therapy for victims of child sexual abuse.

"This defendant spent years sexually abusing a very young girl in vile ways, filming the horrific abuse and then sending it out over the Internet for others with such repulsive interests to watch," said U.S. Attorney David Nahmias.

"The physical and emotional damage he has caused that child is incalculable."