INDIANAPOLIS — Authorities were uncertain today of the source of the carbon monoxide that filled a family's recreational vehicle near the Indianapolis Motor Speedway, killing one person and leaving three hospitalized.

The gas killed 43-year-old Michael Thies and sickened four members of his family inside the RV parked across the street from the speedway, where they were found unresponsive shortly before yesterday's Indianapolis 500.

Lt. Trent Theobald of the town of Speedway's police department said the carbon monoxide did not appear to have come from the family's RV as it was hooked up to an electrical source and its generator was not in use.

"It was a working generator, but they were saying that they didn't run it," Theobald said.

Thies' wife was treated and released from a hospital, but her father, Charles Buss, 69, of Marissa, Ill., and uncle, Fred Buss, 61, of Oakdale, Ill., were listed in serious condition today at Wishard Memorial Hospital. Debrah Buss, who is Charles Buss' 53-year-old wife, was listed in fair condition.

Two others who traveled to the race with the group slept outside the RV, which was owned by one of the Buss brothers, Theobald said.

Bob Fishburn, fire marshal for the town of Speedway, said carbon monoxide poisoning had been confirmed through blood testing on the family members.

Word of Thies' death spread quickly in his hometown of Ruma, a village of just 250 people about 45 miles southeast of St. Louis.

Village president Rick Dugan said Thies had been a friend for three decades.

"I'm just shocked, floored," Dugan said. "You wouldn't think in this day and age that carbon monoxide poisoning would get you."

Thies, the father of four children, was an outdoorsman who hunted and owned a pontoon boat and had been to the Indy 500 "quite a few times," said Russ Hoernis, owner of the Hoernis Auto Body shop where Thies worked since 2000.

"He's just a good all-around guy," Hoernis said. "He just knew everybody, and he was well-liked by a lot of people. Since everyone knew him, everyone trusted him. This is all very sad."

Theobald said no criminal wrongdoing was suspected, but he did not know when a determination would be made on the source of the fumes. He said he did not know whether the RV had a carbon monoxide detector.

The Illinois family's RV was among several parked close to each other in the empty lot just outside the speedway's fourth turn.

Frank Stallion, 42, of Hickory, N.C., told reporters Sunday that he and his wife were in an RV next to the Buss and Thies family. He said investigators told him they were looking at the possibility that his auxiliary generator caused the carbon monoxide fumes.

"I hope we had nothing to do with it," he told The Indianapolis Star. "We've come to the race for more than 20 years and run our generator, but this was the first time we've been so close to another trailer. Usually we are 15 feet apart. This time we were maybe 3 to 5 feet apart. No one thought anything of it."

Stallion could not be located for comment Monday as no telephone number was listed in his name in North Carolina.