Kansas City, Mo. ? About 300 people were evacuated from an industrial park Monday morning after an explosion injured three people at Stratco Inc., a chemical-testing plant.

Two of the workers burned in the explosion were taken to the Kansas University Medical Center, were one was listed in serious condition and the other fair. The third injured person was released after treatment at North Kansas City Hospital. The names of the workers were not released.

Authorities had not determined the cause of the explosion, which happened shortly after 9 a.m. No damage estimate was available.

“We heard a big boom, and we thought a truck had backed into the dock too hard,” said Jerold White, who works nearby at O’Brien Walls and Partitions.

He said that when he looked outside, he saw white smoke and flames. Soon afterward, the smoke turned black, then white again.

Authorities said many chemicals were stored inside and outside the plant, including 90 gallons of sulfuric acid, 1,700 gallons of isobutane and 4,000 pounds of compressed hydrogen gas.

Fire officials estimated that the evacuation involved at least 10 businesses.

About 12:35 p.m., the Kansas City Fire Department sent three teams into the building to take air samples, extinguish fires and check for chemical hazards.

Firefighters and Stratco officials watched nervously from a distance as the teams one at a time entered the 2,500-square-foot, one-story building, the smallest in the industrial park.

Chief Fire Marshal Ed Collins said Stratco officials detailed the building’s layout and said that the fire was in the laboratory. The first team reported heavy structural damage under the roof and spotted a fire at an acetylene tank, which it immediately doused.

Before sending in the teams, Collins said, he had watched white smoke rising from the roof. If the plume had changed colors, it would have indicated that additional chemicals were mixing into the blaze.

Fire officials waited for the fuel to burn before sending in the teams.

By late afternoon, firefighters from five companies had extinguished the blaze and declared the building stable.

Stratco officials declined to comment, saying owner Diane Graham would conduct a news conference upon her arrival in Kansas City from Phoenix.