James Mumper Jr., 19, was arrested about a month after the Nov. 12, 2022, stabbing death of Mateo Castillo, then a senior at Valhalla High School in Rancho San Diego.

Mumper, who was 18 years old at the time of the killing, pleaded guilty to voluntary manslaughter in August. His guilty plea included an allegation of using a deadly weapon in the killing and a stipulation to a 12-year prison term.

Police received a 911 call around 12:45 a.m. on Nov. 12 reporting that someone had been stabbed and was dying in the 900 block of Rigley Street, according to the Chula Vista Police Department.

The victim was found with at least one stab wound and pronounced dead at the home less than an hour later.

Chula Vista police said that in addition to the fatal stabbing, a different 17-year-old boy was stabbed, and another was assaulted during their investigation into the party, which more than 60 people attended.

At Mumper’s sentencing hearing, the victim’s mother, Samantha, said Mateo’s younger brother was at the party and witnessed his brother’s last moments.

Mumper did not make a statement at the hearing, but his defense attorney, Blair Soper, said his client had expressed remorse to him from the outset of the case.

“He wishes to express his sincerest apologies to the family. He knows nothing he can do can bring Mateo back,” Soper said.

Superior Court Judge Michael Popkins called the killing was “a split- second decision” that upended the lives of the victim and defendant, as well as their families.

“This was a tragedy that didn’t have to happen,” the judge said.

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