Thomas Junta claimed he killed Michael Costin, 40, in self-defense after they argued over rough play during the practice on July 5, 2000.
The nine women and three men of the jury deliberated about 13 hours after a weeklong trial that threw a spotlight on the problem of parental violence at youth sporting events and drew national attention in a country where youth sports are both wildly popular and fiercely competitive.
Junta showed little reaction as the verdict was read, though several of his brothers sobbed and hugged other family members as he was led out of the courtroom.
Involuntary manslaughter carries a maximum penalty of 20 years. It is defined as an unintentional, unlawful killing that happened during the commission of a battery that Junta knew or should have known endangered human life.
Sentencing is scheduled for Jan. 25, but as a first-time offender will probably be sentenced to three to five years.
Jurors rejected the more serious charge of manslaughter.
Juror Richard Rotberg, 53, said he sympathized with Junta, but felt he went too far during the fight.
"He acted in self-defense to a certain point. I felt like when he was on top of Mr. Costin, he could have stopped," Rotberg said. "A man was killed," he said. "Someone has to be accountable for it."
Junta testified he tried to avoid coming to blows with Costin, but fought back after the 156-pound Costin threw a "sucker punch" at him, jumped on him and continued to hit and kick him after the two men fell to the floor. Junta claimed he landed just "three off-balance" punches.
But two witnesses testified Junta struck Costin repeatedly in the head while pinning him to the thinly matted floor of the Burbank Ice Arena in Reading. Junta ignored their screams of "Stop. You're going to kill him!" both women said.
Other witnesses, including Junta's 12-year-old son, Quinlan, corroborated Junta's claim that he threw only three punches.
Medical experts for both sides said Costin died of a ruptured artery in his neck, which caused severe brain damage. But they differed sharply on how much force was needed to cause the fatal injury.
Dr. Stanton Kessler, the prosecution's witness, said Costin sustained "severe trauma" from repeated blows to his head and neck. Dr. Ira Kanfer, a defense witness, said a single blow could have caused the artery to rupture.
The fatal fight was witnessed by about a dozen children, including Junta's son and Costin's three sons.
All four boys, ages 11 through 14 - as well as Costin's 11-year-old daughter - sat in the courtroom during closing arguments. Costin's children had been listed as potential witnesses for the prosecution but were never called to testify.
The confrontation between Costin and Junta bgan when Junta became angry about slashing and checking at what was supposed to be a non-contact hockey scrimmage, which Costin was supervising. Junta saw another player elbow his son in the face.
Witnesses said that when Junta yelled at Costin for not controlling the rough play, he snapped: "That's hockey." The two men then got into a scuffle near the locker rooms, but it was quickly broken up by bystanders.
Junta went outside, but returned moments later. He said he came back to pick up his son and his friends, who were still inside the locker room.
Nancy Blanchard, a rink worker, said Junta shoved her aside and headed straight for Costin.
Junta, however, testified that Costin jumped him and he was forced to defend himself. He said he delivered three quick blows, then stopped when he saw Costin put his hand up over his face.
During the trial, the prosecution repeatedly called attention to the barrel-chested Junta's size - 6-foot-1 and 270 pounds. Costin was 6 feet, 156 pounds.
But in his closing argument, Junta's lawyer called him a "gentle giant" who took a "serious thrashing" at the hands of Costin during their first scuffle. Junta had a 4-inch scratch on his face, a ripped shirt, cuts on his arms and scratches on his legs from where he said Costin kicked him with his skates.
Junta, who shed tears at times during his testimony, said he left the rink without knowing how gravely he had injured Costin.
"I thought when he laid back down that he was just resting," he said, his voice choking and chin trembling.
Costin never regained consciousness. He died the next day.
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