A well-known professional mixed Martial Arts fighter has died in San Diego, the San Diego County Medical Examiner's office said.

Kevin Christopher "The Monster" Randleman, 45, of Las Vegas, was in the San Diego area training and attending an event, the ME's office said late Saturday.

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On Thursday morning, he complained of shortness of breath and flu-like symptoms and collapsed, witnesses told the ME's office.

Randleman was pronounced dead on the scene after paramedics tried life support measures, the ME's office said.

Aaron Espinoza, who trained with Randleman at the City of Champions gym, said Randleman did a lot to help veterans and bring light to PTSD after combat. One of his close friends was a Camp Pendleton Marine, Espinoza said, and Randleman spent some time on base last fall to commemorate him.

According to his website, Randleman was a UFC Heavyweight Champion in 2000 to 2001, the IFC Champion Mixed Martial Arts in Brazil in 1998, an IVC Runner Up in 1999. He was inducted into the Sandusky High School Hall of Fame on May 6, 2006, his website says.

He leaves behind four children and a wife, according to a GoFundMe confirmed by a family spokeswoman. As of Sunday afternoon, it had more than $4,200 donations.

The online fundraising page has been set up to contribute to his family and to "share Kevin's passion to inspire and motivate others to greatness," a spokeswoman for the family says.

"As popular Kevin was inside the cage/ring, he was more impactful outside. His words were crisp with honesty, and he had this ability to make anyone believe in themselves," wrote Karen, a family friend, on the page.

Fans shared their memories of the MMA fighter on NBC 7's Facebook page.

"I watched all his fights from the beginning of his MMA career," said NBC 7 San Diego Facebook fan Doug Underwood. "I'm real sorry to see him pass away at a real young age."