ST. CLOUD, Minnesota (CNN) -- Central St. Cloud was recovering Saturday from a natural gas explosion that killed four people and injured 16.

The blast in the central Minnesota town of about 50,000 people was triggered Friday after a construction crew apparently ruptured a gas line while laying fiber optic cable. Investigators had not determined what ignited the leak.

The blast near the Stearns County courthouse flattened a pizza restaurant that was closed for remodeling, apartment units, a law office and a bar, and shattered glass in buildings and cars over several blocks, authorities said.

After touring the scene, Minnesota Gov. Arne Carlson said the area "looked like from the air that a bomb had dropped into it."

"It totally decimated those three buildings," he said.

Two workers from Northern States Power Co. who had responded to reports of a leak -- 53-year-old Karl Klang and 46-year-old Robert J. Jacobs -- were killed in the explosion.

A third victim was a 68-year-old tenant of an apartment above one of the buildings that collapsed, and the fourth victim was a 50-year-old woman who was passing through the area on foot. Their names were not yet released.

Rescue workers searched the rubble into Saturday morning for victims and survivors, but St. Cloud Fire Chief Mike Holman said he was certain everyone was accounted for.

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

'Oh my gosh, someone blew up the place'

Kim Peterson was working in a nearby office late Friday morning when the explosion took place.

"We just heard this huge boom, and the whole building just shook," she said. "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.

"It just took the breath right out of me," said John Donovan, an employee at a theater adjacent to the destroyed pizza restaurant. "we didn't know what had happened. It sounded as if something had fallen -- a building had fallen."

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.

The Minnesota Office of Pipeline Safety sent two inspectors to the scene to investigate the cause of the explosion, and a team from the National Transportation Safety Board arrived from Washington Friday night.

Reuters contributed to this report.