When 17-year-old Jeremy Sanchez's parents learned from administrators at a Southern California high school Wednesday that their son hadn't shown up to class, they turned to one of his friends for help.

It was that friend who, after searching for Sanchez along the San Gabriel River Trail, found his body face down in the bushes by the water, pierced with stab wounds.

And it was that friend - a 16-year-old boy - who sheriff's officers say killed Sanchez.

Wrestling coach Ray Castellanos, far right, and assistant coach, Monique Cabrera, centre, stand next to a makeshift memorial marking the site where Jeremy Sanchez's body was found. Photo / AP
Wrestling coach Ray Castellanos, far right, and assistant coach, Monique Cabrera, centre, stand next to a makeshift memorial marking the site where Jeremy Sanchez's body was found. Photo / AP

"This is somebody who obviously planned this, who tried to plan it well," said Lt. John Corina of the Los Angeles County Sheriff's Department. "And who then tried to throw everybody off."

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Officers on Thursday arrested the 16-year-old, whose name has not been released because he is a minor. He was booked at Los Padrinos Juvenile Hall in Downey, California.

Investigators still don't know the motive for Sanchez's killing, Corina said. While investigators believe Sanchez, a popular student-athlete, was stabbed with a knife, they have not yet recovered the weapon. Corina said investigators aren't sure if there was an argument or problem with their friendship that led the 16-year-old to allegedly kill Sanchez.

It was the 16-year-old's behaviour after he discovered Sanchez's body - as well as statements he made during his interviews with investigators - that led investigators to suspect him. Investigators obtained a search warrant for the 16-year-old's home, where they found sufficient evidence to arrest him, Corina said. He did not confirm whether the 16-year-old boy confessed.

Sanchez's death has shaken South El Monte High School, where grief counsellors were made available to students and where a memorial for Sanchez was planned for Thursday evening.

"Jeremy was a happy, funny and lighthearted student who had a passion for sports, having played on the football, baseball and wrestling teams," school officials said in a statement. "He was a beloved, natural leader on our campus and his death is nothing short of devastating."

Family members gather in a circle of prayer on the riverbed for South El Monte High School athlete, Jeremy Sanchez. Photo / AP
Family members gather in a circle of prayer on the riverbed for South El Monte High School athlete, Jeremy Sanchez. Photo / AP

Sanchez's varsity football coach, Frank Gallardo, told Los Angeles Times that "was a little guy," at just 5 feet 6 or 5 feet 7, weighing about 130 pounds. But he was athletic and had a deep passion for the sports he played, the Times reported.

Gallardo said Sanchez was quick to make people laugh. Since his death, Sanchez's teammates have shared memories of him with their coach, including a video that showed Sanchez rolling down a hill in a tire, emerging dizzy but laughing, Gallardo said.

"There's a big sadness in school. You can see it," he told the Times.