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CASTLE ROCK, Colo. — One of two teenagers accused of opening fire at a Douglas County School will be tried as an adult, a judge ruled Wednesday.

Douglas County District Court Judge Jeffrey Holmes denied a request by the defense to move the case involving 16-year-old Alec McKinney to juvenile court.

RELATED: Judge’s ruling to try Alec McKinney as an adult

The May 7 shooting at STEM School Highlands Ranch left one student , Kendrick Castillo, dead and eight other students wounded.

Castillo and two other students rushed one of the shooters who opened fire inside the classroom.

“I was up at 3 a.m.. I just couldn’t sleep knowing this was coming,” Castillo’s father John Castillo said.

“I am satisfied and pleased that the court agreed that this mass shooting case should be resolved in adult court,” 18th Judicial District Attorney George Brauchler said.

“I am grateful to the victims and their families for the patience and understanding they have shown as they navigate a challenging and often-times slow justice system.”

In September, a judge ruled McKinney’s alleged accomplice, 19-year-old Devon Erickson, could be prosecuted on 44 charges, including murder and attempted murder.

Holmes heard testimony, including from Castillo’s mother, over several days during a reverse transfer hearing last month.

In a seven-page ruling, Holmes considered McKinney’s maturity level and likelihood of rehabilitation, saying “past performance does not provide persuasive evidence that McKinney will take advantage of the services that are furnished.”

The judge noted McKinney frequently failed to attend class at STEM School Highlands Ranch.

During the hearing, defense attorneys tried to paint a picture of McKinney’s troubled childhood, while prosecutors argued the teenager deliberately planned out the attack for weeks.

The Castillos say the decision brings them one step closer to closure.

“In the end, Kendrick is not magically going to appear again,” John Castillo said. “We know that, but the perpetrators of the crime need to be punished.”

By being tried as an adult, McKinney is facing a 40-year minimum sentence in the Department of Corrections if he is found guilty.

Had he been tried as a juvenile, McKinney could not be sentenced to life without the possibility of parole under Colorado law.

McKinney is due back in court on Dec. 16 for an arraignment.

Erickson is scheduled to be arraigned Friday where he is expected to enter a plea.