Michael Meredith, 23, a 2005 graduate of died May 12 of unknown causes, according to officials.
O.C. sheriff's spokesman Jim Amormino said three deputies were called at 10:28 a.m. on Thursday to a home on the 28000 block of Via Faro in Mission Viejo, where a man was reported to be "unresponsive."
"Paramedics administered CPR for five minutes. There was a faint pulse," Amormino said. "He was transported to Mission Hospital, where he later died."
Amormino said that an autopsy has been performed but that it is "inconclusive" at this time.
Long time friend Chandler Kasper, 20, who said he has known Meredith since elementary school was surprised to hear the news. He now lives in Las Vegas.
"My mom and his mom have been best friends since I was in middle school," he said. "Mike was a good guy along with his two younger brothers Will and Andy. He always was nice to everyone."
Kasper said Meredith liked to exercise a lot from what he recalled.
"The last time I talked to him was last week on a Facebook post," he said. "He put a heart. I was more friends with his brothers because I am a lot younger. Andy and Will came to Vegas about two weeks ago."