“One of the gentlemen that was downstairs saw a bright flash of light,” Waterman said. “He alerted everyone and pulled the fire alarm.”

A Nosler worker told us off-camera he didn't even realize anything was wrong, He thought it might have just been a fire drill when everyone was evacuated.

But once he got outside, he quickly realized something was wrong. He saw the big orange plumes of smoke coming out of the building. Then the explosion happened, and how he described it was, the roof rose up and then fell right back down.

Witnesses say the explosion was felt by people all over the immediate area. some as far away as a half-mile or more.

Much of the area around the building was closed off Wednesday because there are potentially still some combustible materials inside that could set off another explosion.

Investigators from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, along with Oregon State Police and Bend fire investigators, gathered at the blast scene Thursday morning, working to figure out what triggered the explosion.

Bond said fire crews arrived to find extensive damage to the southeast corner of the building and brownish-yellow smoke pouring from the damaged area.

A fire special rescue operations team combed the wreckage to be sure all workers were accounted for, while city building officials helped evaluate the damage and hazards for crews performing mop-up and overhaul operations.

Bond also said the quick evacuation by workers was "instrumental in a safe and effective exit from the structure."

"The company instituted and practiced building evacuations over the years, and this policy paid off today," Bond said in a news release.

A damage figure still was being compiled for a facility valued at $2.8 million for the structure and $10 million worth of contents, but Bond said it clearly was "pretty extensive."

Columbia Street remained shut into the evening hours. Firefighters asked the public to avoid the area for their safety and that of emergency workers.

Bend Fire did a personnel callback to ensure adequate staffing and called in Redmond and Sunriver firefighters for mutual aid coverage of the North and South stations.

The explosion buried at least one evacuated worker's car in debris.

One nearby resident described the blast as like "feeling a bomb inside your house." After her house shook violently, she looked out to see a huge plume of orange smoke.

Barb Gonzalez at first thought it was an earthquake, when her house was jolted by the blast. "This one felt like a 5.5 (magnitude) jolt", said the survivor of the 1994 Northridge quake in California. Her second thought was an explosion.

People several blocks away said the explosion rattled their office windows.