Kansas City, Mo. ? A massive explosion at a chemical distribution plant that sent flames towering into the sky and shrouded Kansas City in black smoke has been ruled an accident.

Fire Chief Richard Dyer said the blaze started Wednesday when employees were offloading a petroleum-based product. Several 55-gallon drums ignited, touching off a series of explosions in the mixed residential and industrial East Bottoms neighborhood.

Nobody was killed in the blast, but two of the nine workers who had been inside the Chemcentral facility suffered minor injuries.

The fire department determined that conditions were safe enough late Thursday to allow most residents and business owners in a six-block restricted area to return.

The Kansas City Police Department planned to limit traffic in the area to protect people still working at the site, Dyer said. Residents and business owners will be allowed to pass checkpoints with proper identification.

The site was turned over to company officials Friday afternoon, when it was determined that all fires had been extinguished and there was no sign of toxic vapors. The Environmental Protection Agency and the Missouri Department of Natural Resources have said they will continue to monitor the area for potential environmental effects.

The fire destroyed the five-acre plant, but company officials said they plan to rebuild in Kansas City. Losses are estimated at about $1 million.