TULSA, Oklahoma - Two employees are recovering after an explosion at the Honeywell Aerospace plant in Tulsa Tuesday morning. 

The explosion blew a 30-by-60-foot hole in the top of the building. Fire crews said it could've been much worse.

"People thought it was thunder. And after the explosion, we saw the big cloud of black smoke," said ISTI Supervisor George Gonzalez. "We knew there was something, something wrong."

The Honeywell Plant at 6930 North Lakewood Avenue, just around the corner from where Gonzalez works, was on fire.

Investigators said a paint booth inside the building exploded. The blast was heard miles away.

"After that, we saw a whole bunch of firetrucks," Gonzalez said.

The first thing emergency crews saw was the hole in the ceiling of the plant and thick black smoke.

Firefighters went in first, hazmat and heavy rescue crews followed.

"They could hear a lot of gas flowing through. One of the pipes had broken, had a lot of arching," said Captain Stan May.

May said the fire was out after 20 minutes; most of that time was spent waiting for specific gas lines to shut off.

"We didn't really want to shut the whole building down because they have exhaust fans and other safety features in place that we wanted to use," he said.

Once crews contained the fire, they were able to assess the damage and make sure the structure was safe to turn back over to the owner.

May said the building's design helped control the flames.

"They had blast walls in place, firewalls in place to slow down that blast from allowing the fire and the pressure to get out into the rest of the building,” he said. "It cost them part of their roof, but again, it's better that than pressure and heat go up through the roof, then it would be for it to go in the rest of the plant."

The two people injured are expected to be okay.

Firefighters believe the explosion was an accident. It’s up to the company to investigate further.