An explosion in a Milwaukee suburb demolished a church and damaged two homes Wednesday, injuring seven people, including three firefighters, authorities said.

Road and sewer work was being done in downtown Oconomowoc before the explosion occurred around 1:30 p.m., said Bob Duffy, the city's economic development director. The blast and flying debris knocked over several workers, he said.

WE Energies had received a call from a contractor about an hour earlier, saying workers smelled gas and may have hit a line, company spokesman Brian Manthey said. The company sent out a crew, and at least one of its workers was injured, he said.

Another WE Energies spokesman, Rick James, said he did not yet know whether natural gas caused the explosion. But the company turned off natural gas service to about 150 customers in the area around First Baptist Church Oconomowoc, and emergency workers were evacuating nearby homes and businesses.

Two investigators from the Occupational Safety and Health Administration were sent to the scene, said Christine Zortman, assistant area director in Milwaukee.

One person was flown to Froedtert Memorial Lutheran Hospital and was being evaluated, hospital spokeswoman Carolyn Bellin said. Four others, including a firefighter, were taken to Oconomowoc Memorial Hospital with cuts, bruises and abrasions, administrative assistant Jeanne Ward said.

Duffy said two other firefighters suffered minor injuries, but he did not know their whereabouts.

Smoke billowed from what remained of the church and a neighboring home Wednesday afternoon, obscuring the view of the 100 or so people who had gathered outside.

Joy Freudenstein, 30, was cleaning her house about three blocks from the church when she heard a boom. She immediately thought it was an earthquake.

"It shook my house," Freudenstein said. "It sounded like someone downstairs blew something up."

Waukesha County Board Supervisor Kenneth Herro, who works a block away as a real estate agent, said he heard an explosion and walked to the scene.

"The church basically had disappeared and the only thing remaining was the steeple in the front," Herro said.

He said he smelled natural gas and ambulances were pulling away as he arrived. One house on each side of the church was on fire, he said.

The explosion shook downtown, Herro said. "We have really large windows, all the windows just kind of bowed out," he said.

Oconomowoc is about 30 miles west of Milwaukee.

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Associated Press Writer Carrie Antlfinger in Milwaukee contributed to this report.

(This version CORRECTS that four people were taken to hospital).)