Kansas City, Mo. ? An investigation has begun into what touched off a massive explosion at a chemical distribution plant on Wednesday, causing the evacuation of several hundred people from a mixed residential and industrial area.

The explosion at the Chemcentral Corp. plant in Kansas City’s East Bottoms neighborhood sent fireballs towering into the sky and caused an ominous plume of black smoke to drift over the downtown skyline.

Robert Garner, senior vice president of environmental affairs for Chemcentral, spent Thursday surveying the site as firefighters continued to douse the last of the hot spots. He said an environmental team is ready to begin its investigation of the site.

“We expect to work hand-in-hand with the United State Environmental Protection Agency and the Missouri Department of Natural Resources,” Garner said.

Fire Chief Richard Dyer said the department is cooperating with federal agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives to determine the cause of the blaze. But he declined to give a timetable for the investigation and said any cause will likely be based on interviews with witnesses because physical evidence was destroyed in the fire.

Dyer said preliminary indications are that some chemical was released in the plant and ignited, setting off additional explosions.

Garner said the chemical was probably polybutene, a relatively safe chemical that is used in a variety of products, including liners for things like cereal boxes.

“It’s a product that we’ve handled for 30 years, always very carefully, safely,” Garner said. “Employees were following the procedures and precautions.”

Garner said employees in the plant looked up to see a fire breaking out Wednesday afternoon. The plant was evacuated before a series of 55-gallon drums ignited and touched off the first explosions. Nobody was killed in the blasts, but two workers at the plant suffered minor injuries.

Officials had planned to allow the fire to burn out on its own, but it had mellowed enough by late evening to allow firefighters to approach the site and apply flame-suppressing foam. Dyer said a minor explosion occurred Thursday morning as smoldering embers touched off residual chemicals, but nobody was injured.

Firefighters continued to watch for hot spots late Thursday.

Most people who were evacuated from neighborhoods surrounding the site have been allowed to return, deputy fire chief Frank Tittone said. But a mandatory evacuation remains in effect for homes and business within a half-mile radius of the plant. No timetable was given for when they might be able to return.

Dianna Whitaker of the Environmental Protection Agency said a plane that was sent over the city to collect air-quality samples on Wednesday found traces of trimethlybenzene, ammonia and methanol, but all of the chemicals were well below health-based benchmarks.

The EPA is continuing to monitor air quality at several locations throughout the metropolitan area, Whitaker said, but the level of toxins released by the explosions all appear to be within acceptable levels.

Whitaker said she is unsure when the EPA will be allowed into the site or what the agency’s next step will be.

“We’ve not made that determination, but when we do, we will coordinate with the city and state,” she said.

Dan Brennan, an attorney for Bedford Park, Ill.-based Chemcentral, said the plant has 15 to 20 workers, nine of whom were in the building at the time.

Company officials said that it is too early to assess the future of the company in Kansas City. Customers who were served by the distribution center are receiving materials from warehouses in St. Louis, Springfield and Tulsa, Okla.

“The workers themselves, I don’t know if we’ll be able to move all of them,” Garner said. “But some of them are on their way to St. Louis, helping to supply customers out of that location.”