Daniel Yuen walked out of his boarding school and never returned

After Teen Son Went Missing From Calif. Boarding School in 2004, Mom Says ‘Part of Me Is Missing’

Daniel Yuen, then 16, had always been a happy kid and a beloved part of his tight-knit New Jersey family. But in 2004, the high school junior fell into a deep depression his mom believes was triggered by a breakup with his girlfriend.

“It got so bad, he stopped going to school,” 59-year-old Lisa Yuen recalls to PEOPLE.

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A psychiatrist recommended the Yuens send Daniel to an upscale therapeutic boarding facility called CEDU High School in the California mountains. “His eyes opened up and he said, ‘That would be a nice school!’” Lisa remembers of the moment when she first showed Daniel the CEDU brochure.

But on Feb. 8, 2004, less than a month after arriving on campus, Daniel left the school and hasn’t been seen since.

School administrators told Daniel’s parents he’d simply walked out of the school and failed to return. “We were so devastated, we booked an overnight flight,” Lisa says.

The Yuens returned home to New Jersey without their son, and with no leads on where Daniel was. Daniel had been classified as a runaway by police, which the Yuens believe made the investigation less urgent.

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According to the National Center for Missing and Exploited Children, there were 424,066 reports of missing children provided to U.S. law enforcement throughout 2018. While some of those children may have run away, others may have been abducted or worse.

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In the 15 years since Daniel vanished, his devastated parents have hired private investigators, but their heartbreak continues over their son, who would be 31 now. Lisa says there have been a few possible sightings but still no concrete answers. “Part of me is missing,” says Lisa. “If people see him, at least it gives me some hope.”

Lisa says Daniel’s disappearance has been especially tough on his sister, Selena, who is four years younger. “She looked up to him,” Lisa remembers, noting Selena’s “nonstop crying” after her brother went missing. “He was an active big brother and a very caring person, always polite.”