Firefighters look for any trapped victims

ST. CLOUD, Minnesota (CNN) -- A natural gas explosion rocked the downtown area Friday, killing three, injuring at least 20, and possibly trapping others in the rubble. The blast flattened a restaurant, severely damaged adjacent buildings, and shattered glass in buildings and cars over several blocks, authorities said.

Rescue workers were looking for survivors buried in the debris, and authorities evacuated much of the area, including the two-block Courthouse Square.

Cloud Hospital was treating the injured, and a spokeswoman said several were in critical condition.

The explosion is believed to have been caused when construction workers laying cable in or behind a closed pizza parlor accidentally struck a natural gas line, according to Mayor Larry Meyer and a spokeswoman for Gov. Arne Carlson.

An ambulance on the way to the scene

At 11:25 a.m. CST (12:25 p.m. EST), it damaged four adjacent buildings and shattered windows in a courthouse across the street and other buildings, the mayor said.

"We just heard this huge boom, and the whole building just shook," said Kim Peterson, who was working in an office nearby.

"Everybody just ran to the window and saw this huge cloud of black smoke. I was shaking, I was so scared.

"We thought it was a bomb at first.... We thought, 'Oh my gosh, someone blew up the place,'" she said.

The Minnesota Office of Pipeline Safety sent two inspectors to investigate the cause of the explosion in this city of 50,000 about 70 miles northwest of Minneapolis.

"We have a report that it was a gas pipeline," said Charles Kenow, an agency administrator. "But at this point all we have is information on people smelling gas on scene, and we need to investigate that further."

Fire departments from at least three other communities were called in as backup, and Northern States Power Company crews disconnected electricity in the area as a precaution, said spokeswoman Mary Sandok.

Emergency rescue dogs have also been brought in to help locate the missing, authorities said.

Reuters contributed to this report.