Big wave surfer Adam Francis D’Esposito, known as “Biff,” was found dead after he was reported missing in Baja, Mexico nearly three weeks ago, his family confirmed this week.

Mexican officials recovered his body in Rosarito, Mexico, 30 miles south of San Diego, on Sept. 9, but D’Esposito’s family only learned of his death Wednesday, according to a Facebook post from his sister, Briana D’Esposito.

Mexican coroners determined the cause of death was drowning, according to Fox 5 San Diego.

But the surfer’s family is disputing that account, noting that D’Esposito was an unusually strong swimmer who spent years in the water in some of the world’s toughest surf. He was known to frequent big wave hot spots in Tahiti, where he survived a 2012 shark attack.

"Since my brother is a big wave surfer, spending the better part of the last 20 years in Tahiti surfing some of the most dangerous waves in the world, it’s very hard for us and many of his friends to believe he would just drown. It seems like there has to be some more to the story that led him to that point,” his youngest sister told NBC 7 San Diego.

D’Esposito, 39, lived in Carlsbad, Calif., with his 6-year-old daughter. He left for Mexico on Sept. 2 for a surfing vacation with his father. He borrowed his silver Volvo station wagon on Sept. 6 and did not return.

He was spotted running on a highway between Rosario and Tijuana without shoes or a shirt two days later, according to the NBC station.

His family fears mental illness could be a factor in D’Esposito’s death. He was diagnosed with bipolar disorder in February, Briana D’Esposito told CBS News 8 San Diego.

D’Esposito’s disappearance drew worldwide attention in the close-knit surfing community. Champion surfer Kelly Slater posted a missing poster on Instagram Sunday to encourage anyone with information on D’Esposito’s whereabouts to contact authorities.

“It really hits home when someone you know goes missing. Please, anyone with any info whatsoever call the numbers on this post and/or leave a message below,” Slater implored in the post.

Memorial arrangements are pending, Briana D’Esposito wrote on Facebook. The family has arranged a GoFundMe page to raise money for the surfer’s daughter and funeral expenses.

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