A California father who vanished Saturday while on an 8-mile run in a regional park during sweltering temperatures is either "incapacitated" or no longer in the area, authorities said.

Philip Kreycik, 37, parked his car in the Moller Trail staging area at Pleasanton Ridge Regional Park around 11 a.m. before going missing, the Alameda County Sheriff’s Office said. Temperatures reached 106 degrees that day. The park is located outside the city of Pleasanton in the East Bay Area.

MISSING MONTANA HIKER BELIEVED DEAD, SEARCH SCALED BACK: AUTHORITIES

Hundreds of volunteers and professional rescuers joined the search after Kreycik’s family reported him missing Saturday afternoon. Agencies have deployed search dogs, drones, helicopters and an airplane with advanced thermal imagery.

But after four days, there is still no sign of the missing runner, whose cellphone and ID were found inside his parked vehicle. Officials announced that the massive search would be scaled back.

Hundreds of volunteers and officials using drones, aircraft and off-road vehicles searched Tuesday for Philip Kreycik, who went for a run near  the San Francisco Bay Area and didn't return home.

Hundreds of volunteers and officials using drones, aircraft and off-road vehicles searched Tuesday for Philip Kreycik, who went for a run near  the San Francisco Bay Area and didn't return home. (Pleasanton Police Department / Bay Area News Group)

"We’ve come to two conclusions: that he’s either incapacitated within the search zone and we can’t find him; or he’s not here, and he’s somewhere else that we don’t know about," Sgt. Ray Kelly said during a press briefing on Tuesday. 

Police described Kreycik, a father of two, as "an endurance athlete" who is in top physical condition and "thrives in extreme environments." Friends told KGO-TV that Kreycik has no known health conditions.

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"We are saddened that our efforts thus far have not located him yet," the sheriff’s office said in a statement. "We remain hopeful but the lack of results are frustrating to all of us on this mission."

Kreycik is an energy efficiency analyst for Pacific Gas and Electric, the utility confirmed to The Associated Press. He graduated from Harvard and MIT, and runs ultramarathons, police told KGO-TV. 

Authorities asked anyone with information about Kreycik’s disappearance to call the Pleasanton Police Department.

The Associated Press contributed to this report.