Jurors deliberated for more than 10 hours over two days and convicted him of reckless homicide.
"I'm just shaking," McLean said moments after the verdict was read. "I feel hopeful, and I feel hopeful again for my children that they can have a good life."
McLean had faced up to life in prison if convicted of first-degree murder. Instead, reckless homicide carries a sentence of two to four years in prison, but his attorney said he will fight for no prison time.
"The plan is that Eric will apply for probation," defense attorney Bruce Poston said. "Remember someone still died and this man will never forget that. He is always going to be marked."
Sentencing was scheduled for Nov. 7.
Prosecutors said it was a premeditated killing that McLean planned at least two weeks earlier when he stole the 30-caliber rifle and three bullets from his parents' house. McLean said he took the gun and bullets to commit suicide.
Assistant District Attorney Bill Crabtree said he was convinced McLean helped his case with his own testimony. McLean described the shooting as an accident in which he aimed a high-powered rifle at Powell only to scare the teenager, described by the defense as "mature beyond his years," and the gun fired when Powell grabbed the barrel and McLean pulled back.
Prosecutors insisted they didn't overreach with the first-degree murder charge.
"The proof was there, it was up to them (the jury) to make the decision," Crabtree said.
The families of both McLean and Powell wept at the end of the weeklong trial.
"This is like a rebirth almost of our family — to have Eric back, to have Eric out of this situation," said his mother, Tonya McLean. "I feel compassion for the Powells. It has been hard on them, too."
Scarlett and Jack Powell, who took in Sean as a foster child and later adopted him, were crushed by the verdict, Crabtree said.
"They loved this boy. He was their son and he was killed," Crabtree said. "They just feel by this verdict the jury did not think much about the value of Sean Powell's life."
McLean hasn't seen his young sons, Eric Jr. and Ian, since last year when they disappeared with McLean's now ex-wife, Erin. Prosecutors said they still don't know where Erin McLean is, but Poston vowed he will try to find her.
Erin McLean met Powell when she was an intern teacher at his high school in late 2006. Eric McLean became suspicious of phone calls and text messages between the teen and his wife of 10 years. He discovered them meeting in secret and ultimately caught them having sex.
But every time he tried to get his wife to break it off, she got mad at him and demanded Powell be welcomed into their home, McLean testified. He said he still loved her, so he agreed.
On the day Powell was shot to death, he had come to the McLeans' house and Eric McLean tried to get him to leave. Powell went to his car and waited for his English teacher to join him.
Eric McLean testified he then went to the car and the rifle accidentally fired. McLean was arrested the next morning aimlessly walking along railroad tracks.
His first words to the officer: "Last night was a mistake."
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