A petroleum gas leak is suspected of triggering a Friday morning explosion in the laundry area of a Willowbrook condominium complex that sent one woman to the hospital and causing at least three separate house fires and numerous smaller explosions that may have injured a second person, authorities said.

Authorities said the last of the fires was reported around 1 p.m., but more were possible.

Tri-State Fire Protection District Chief Dan Niemeyer said investigators believe fuel leaked from a Speedway station at 6241 S. Cass Ave. just north of 63rd Street in Westmont into the Flagg Creek Water Reclamation District sanitary sewer system and local storm sewers.

Officials said there have been more than 100 reports of noxious odors in the area and at least 10 flashes and/or explosions and fires -- mostly along 63rd Street between Cass and Route 83.

Deputy Chief James Connolly of Westmont said firefighters found gasoline in the sanitary sewer directly across from the gas station about 10:30 a.m. Friday.

"That was quickly isolated to prevent additional gasoline from entering the sewer," he said.

There is a continued risk of fires and explosions, though, because gasoline remains in the sewer that runs for about 8 miles along 63rd Street to the water reclamation district plant.

Crews are working to ventilate the sewers, insert an additive to dilute and mitigate the gasoline that's still in the system, and add foam to reduce vapors, but authorities urged residents to immediately call 911 if they smell gas.

"It's a real problem," Niemeyer said. "It's going to take a while to mitigate. We have to monitor individual houses and sewer systems from here to what we believe to be the source and beyond."

Connolly said Friday night Speedway and their contractors have removed approximately 60,000 gallons of the water and gasoline mixture.

"We believe we have the source of the problem isolated," he said. "We still have fire companies patrolling that entire area, ventilating the system, tracking product and putting emulsifier in the system to reduce any vapors."

The explosion left one building in the Knolls of Willowbrook condominium complex on the 6200 block of Knollwood Drive uninhabitable, authorities said. Utilities have been shut off to the other buildings.

House fires, meanwhile, were reported earlier in the day on the 6200 block of Bentley Avenue, the 300 block of 63rd Street and the 300 block of Coralynn Court.

Niemeyer said crews are checking area sewers and, if they find gasoline, are branching out to individual homes. He said he has no idea when condo residents will be allowed to return home permanently.

The initial explosion happened around 9:30 a.m. in a laundry facility in a common area of one of the condo buildings, authorities said, and caused extensive damage.

The fire was extinguished quickly but the odor continued to linger.

A woman who was in the area was taken to Hinsdale Hospital and Niemeyer said "apparently she's doing OK." She was identified as 80-year-old Margaret Rice, ABC 7 Chicago reported.

A second person was reported injured in one of the other fires or explosions, but details were not immediately available.

Authorities said the Tri-State Fire Department first investigated a report of an odor around 2:30 p.m. Thursday at 6109 Knoll Valley Drive.

Willowbrook police said they received a report of a gasoline smell at 3:41 p.m. Thursday and then firefighters investigated seven more reports in the next seven hours.

Willowbrook police Chief Robert Pavelchik said authorities couldn't determine the source of the odor Thursday night.

"It was only a smell at the time," he said. "It wasn't deemed unsafe."

A resident of the condo complex, Crystal Sheley, waited in the parking lot late Friday afternoon with a pet crate to retrieve her two cats. She said she's neighbors with the woman injured in the explosion.

Sheley said she smelled a strong odor before leaving for work about 7 a.m. and complained of a having a headache throughout the day.

"It was suffocating," she said of the stench.

Residents thought the odor smelled like paint fumes and initially chalked it up to crews painting balconies at the rear of buildings in the complex.

"It just seemed kind of coincidental," she said.

Residents also heard fire alarms around 3 p.m. Thursday and again at 4:30 or 5 p.m.

The American Red Cross was contacted to provide aid to those who have been evacuated.

Authorities said they will remain on the scene until EPA investigators tell them the sewer line is free of gasoline and fumes.

• Daily Herald staff writer Justin Kmitch and ABC 7 Chicago contributed to this report.