Kiel, 27, was driving alone after leaving a party at about 10:15 p.m. PT Friday night when he hit a wall in San Diego's upscale Scripps Ranch neighborhood and was thrown from the car, police Sgt. Alan Hayward said.
Kiel was barely breathing when paramedics reached him and he died about an hour later, Hayward said.
Friends had tried to keep Kiel from driving home from the party, Hayward said, and witnesses told police he appeared to be driving in the wrong direction when he crashed.
Police would not know whether Kiel had been under the influence of drugs or alcohol until toxicology tests were performed, Hayward said.
A second-round draft pick out of Texas A&M in 2003, Kiel played four years in the NFL from 2003 to 2006, all with the Chargers.
In February 2007 he pleaded guilty to felony and misdemeanor drug charges for shipping prescription cough syrup to Texas, the most significant of several scrapes with the law.
He was released by the team after his plea.
Kiel had been led off the practice field and arrested in the locker room by Drug Enforcement Administration agents in September 2006, suspected of shipping at least two parcels of prescription cough syrup, apparently to be mixed with soft drinks to make a concoction known as "lean."
The felony charges were later dropped after he completed community service, and he was serving three years' probation for the misdemeanor charges.
He was cited for urinating in public outside a San Diego nightclub in January 2007, but the city dropped that charge.