Ivan Pravilov, 49, was found unresponsive in his cell at 3 a.m. Friday, according to spokesman Darrin Howard of the Federal Detention Center in Philadelphia.
A preliminary FBI investigation suggested the death was a suicide, Howard said. An autopsy is pending.
Pravilov was a mentor to several NHL and U.S. college players, including Dainius Zubrus of the New Jersey Devils, who played for him as a teen in Ukraine.
Other former players have said his methods included physical, emotional and sexual abuse.
"For 10 years, he brainwashed us every single day," former player Maxim Starchenko said Friday after learning of the death. Starchenko, 33, of suburban Detroit, wrote a recent book about Pravilov called "Behind the Iron Curtain: Tears in the Perfect Hockey 'Gulag.'"
"He got away with it too easy. I wanted to look him in the eye in the courtroom and just ask him, 'Just what were you thinking all these years?'"
Pravilov ran an elite hockey school in Ukraine from the 1980s until about 2007, when he came to the U.S. to run camps for standout players from the U.S. and abroad. Players from Ukraine and elsewhere typically enrolled for about a month, staying with host families and traveling with Pravilov to various U.S. cities for tournaments and clinics.
On Jan. 3, he allegedly brought two 14-year-old Ukrainian boys to his Philadelphia apartment from a family home in Wilmington, Del., and fondled one of them during the night. The other boy was later threatened in a locker room, authorities alleged. A host parent contacted police.
Pravilov had been in custody since Homeland Security agents arrested him last month. He was later indicted on charges he took the teen across state lines for sexual purposes.
Pravilov had pleaded not guilty. He had appeared animated in several court appearances, speaking to his lawyer through a translator, and telling the judge he had been unable to call his lawyer to work on his case from the prison. He waived his right to bail at his last hearing on Feb. 1, perhaps because he was wanted by Interpol.
The warrant stems from a 2007 fight involving several men in Ukraine.
U.S. prosecutors considered Pravilov both a danger and a flight risk. But he was not deemed a suicide risk, and was being held in a two-man cell in a regular prison unit. The last head count was at midnight, although guards roaming the unit might have seen him alive later, Howard said.
Defense lawyer Mark Wilson spent four hours at the prison seeing other clients Friday afternoon without learning of Pravilov's death. Lawyers, like other visitors, cannot bring cellphones inside. He did not get the news until Friday evening.
"It's shocking, but it's not in some ways. He was not happy about his situation," Wilson told The Associated Press.
Still, he and Pravilov planned a strong defense, and he did not consider him a suicide risk. He said the cause of death was not yet clear.
"We had a story to present," said the veteran public defender, who declined to elaborate on their strategy.
The U.S. Attorney's Office in Philadelphia, which was prosecuting Pravilov, declined to comment.
Serhiy Ivanchov of the Consulate General of Ukraine, said officials there had no comment as they tried to reach family members. The Philadelphia Medical Examiner's Office, which was conducting the autopsy Friday, was also trying to reach family members, spokesman Jeff Moran said.
Pravilov was not married and had no children, according to Zubrus' mother, Irene Zubriene. Pravilov used her Cherry Hill, N.J., home as a mail drop and visited her weekly before his arrest. A close friend, she has blamed the arrest on lingering rivalries from Ukraine.
Zubriene did not return messages Friday. Her son did not return messages left with the Devils.
