An explosion injured four employees at UTC Aerospace Systems in Vergennes Friday. Firefighters have not said what sparked the explosion.

A woman who was across the street at the time told WCAX News she was terrified.

It was an ordinary morning selling Christmas trees for Robin Little-Lebeau. Then, shortly before noon:

"All of a sudden, this noise. It sounded like a giant backfire. I mean, a gigantic backfire. And the whole camper shook. And I thought a tree had fallen on the camper. I literally was petrified," Little-Lebeau said. "And I opened the door and I heard the guys at the rescue hollering."

They were focused on UTC Aerospace Systems just across the street.

"When I looked over, there was this huge plume of smoke. It started out gray but then turned black. And it just kept billowing out," Little-Lebeau said. "And I thought, oh my God, I hope nobody is hurt!"

Four people were taken to the hospital. One was taken to the UVM Medical Center for burns.

Employees evacuated as firefighters and police responded, shutting down the road. The response came quick thanks to inside help.

"There was a loud bang and then the fire alarm went off, and I went into my deputy chief mode," Vergennes Deputy Fire Chief Matt Fraley said.

Fraley is a longtime employee at UTC Aerospace, along with several of his firefighters. He says they train for situations like this.

"It was pretty fast. A lot of training goes on here on evacuations and stuff, so it was done very well and very organized," Fraley said.

UTC manufactures sensors and systems for aerospace and defense industries. All early indications point to an industrial accident. Police told us it happened in a dust collector outside the building and caused internal damage, as well. The extent remains unclear as firefighters investigate.

"Right now, everything remains under investigation," Fraley said.

That investigation continued into the evening. Friday evening, the company released a statement saying some of its 775 workers would be able to return to the job Saturday in unaffected areas of the site.

State police say the explosion is not considered suspicious.