A jury also found he was driving under the influence and fled the scene after hitting Michelle and one of her friends.
On Friday, Mostrong sat in his orange-colored county jail coveralls flanked by defense attorneys Robert Espinosa, who was Mostrong's attorney during the trial, and Eric Beaudikofer, who was added to the defense team after the conviction.
At the start of the hearing, Contreras ruled against a request by the defense for a new trial.
The defense then focused its efforts on trying to keep Mostrong, 21, out of state prison, something they said was not warranted for a young man who had no prior criminal record.
"This is not a life worth throwing away," Beaudikofer told Contreras.
Beaudikofer added Mostrong was in a vehicle with a group of youths when the van struck Michelle. He said three youths bolted from the scene and ran back to the "keg" party they had been before getting into Mostrong's van. Not one, Beaudikofer said, stopped to assist Michelle.
Beaudikofer said of Mostrong, "When he stopped he was in shock. He knew something bad had happened and he couldn't confront it."
Beaudikofer added that even though Mostrong fled, he later turned himself in to authorities.
"I'm not minimizing it," Beaudikofer said of Mostrong's fleeing. "I'm asking the court to see that it was understandable."
Beaudikofer further said people have misunderstood Mostrong's silence throughout the case. He said if Mostrong did not speak about his feelings of remorse, it was because he was advised to not do so.
"Silence may be misunderstood, but it is never misquoted," Beaudikofer said, adding people need to know Mostrong does feel remorse and grieves for the death of Michelle.
Before the sentencing, Mostrong was given a chance to address Michelle's family.
He told them his attorney said he could not call the family, but he wanted to call to say he was sorry.
"I truly am sorry," he said, facing the family. "I never meant for this to happen. I would do anything to bring her back."
He added, "I hope that some day you will be able to forgive me in your heart."
Mostrong then sat down, stating he couldn't speak anymore.
Contreras then gave Michelle's mother and father a chance to address Mostrong.
Marylin Boecher, Michelle's mother, said "eventually" she will be able to forgive Mostrong, but she wants him to know he had taken "the most important thing" away from her.
"I structured my whole life around her," she said. "She was the most incredible daughter."
She added, "How can I express to you what losing her has done to my life?"
Michelle's father, Raymond Grady, said, "This is our life. This is our immediate family. I'm angry because if Blake made a mistake, he didn't care."
Grady said if Mostrong felt remorse, he should have come forward before the case made it to a trial rather than let the case continue into a trial lasting six weeks.
Grady asked that Mostrong get 15 years in state prison, the maximum sentence he could be given. He said even if did get that sentence, it wouldn't compare to the 60 to 70 years his daughter lost.
In pronouncing the sentence, Contreras said all those involved in the events leading up to the death of Michelle compounded one bad choice after another. He said both Mostrong and those he was with and Michelle and her friends made bad choices.
Contreras told Mostrong what happens with the rest of his life is his decision. It is up to him to decide whether he comes out bent and broken from prison.
Contreras pointed out that Mostrong has said he has built a new relationship with God.
"If you have found the spirit of God, you will not come out broken or bent," he said. "That has to come from within you."
After the sentence was announced, Espinosa filed a motion that his client be released on bail pending an appeal, which the defense will seek.
Contreras denied that motion.
The nine-year sentence includes four years for the count of vehicular manslaughter and a five-year mandatory enhancement for fleeing the scene.
He could be eligible for parole after serving half the sentence, according to the District Attorney's Office.
Staff Writer Darren Simon can be reached at 337-4082.