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A 13-year-old boy who prosecutors say was playing with toy trucks and planes right before he shot and killed his parents was sentenced Wednesday to seven years in juvenile detention after pleading guilty to murder.

The boy was 12 in March when prosecutors say he shot Marilyn and Charles Long with a .357 Magnum revolver in Burlington, a small farming community about 140 miles east of Denver. He also pleaded guilty to attempt to commit first-degree murder and three counts of assault for attacking two of his younger siblings.

He could have faced decades in prison if he had been tried and convicted as an adult.

The light sentence angered some relatives, but District Attorney Bob Watson said state law forced him to choose between what he saw as an unacceptably light juvenile sentence and an unnecessarily harsh lifetime sentence.

Watson told a judge there was no explanation for the slayings. He showed photographs of the family's backyard, where he said the boy had been playing with toy trucks and planes minutes before the shootings. He said tests showed that he was immature and noted that money he earned cleaning a church and the courthouse before the slayings had been spent on Legos.

"If you're looking for an adult explanation for why this kid went from playing in dirt to commit murder you'll never get one. This lies in the mind of a very immature 12-year-old," Watson said, referring to the boy's age at the time of the slayings.

The prosecutor said he was also concerned that a judge might have rejected a proposal to transfer the case to adult court and whether the state would be able to get a conviction there, noting that juries like to know why a crime was committed.

Charles Long's brother, Wally Long, said he was never in favor of a plea agreement.

"My desire for a longer sentence was never about hatred or anger but out of a sense of justice," he said.

The boy will also have to serve three years on parole after being released.

Marilyn Long, 50, homeschooled her kids and ran the children's ministry at the Evangelical Free Church. Charles Long, 51, served as an elder at another church and was a snack delivery driver. The boy was a greeter at the church and helped other children memorize Bible verses.

Prosecutors allege the boy stabbed and shot his 9-year-old brother and injured his 5-year-old sister with a knife. Both recovered from their wounds.

In court, the boy stood about 5 feet tall and appeared slight in his teal prison uniform. He declined a chance to speak and cried as a defense lawyer read a statement from his maternal grandmother, Dolores Richardson.

She noted that he was a helpful boy and hinted that the slayings had resulted from "helplessness, a cry for help for a situation, that (he) felt he had no control over."

After the hearing, Wally Long and the boy's 25-year-old brother said they didn't know to what she was referring. "He's dead to me," the brother said.

Before the sentencing, public defender Tom Ward said in court that the boy had made progress while in juvenile detention. He grew two inches, gained 16 pounds, and formed relationships with attorneys, staff and other juveniles, Ward said.

Ward also said the boy has shown "incredible remorse" for the crimes.

"Life in prison is not the proper place for a 12-year-old boy, especially this one," he added.