TULSA, Oklahoma - Fire officials are investigating why a Tulsa business that caught fire Tuesday night. It happened just after 9 p.m. off 69th and South Sheridan.

Employees salvaged their belongings strewn across the parking lot of the Petroleum Equipment Institute lot Wednesday, after the building was destroyed.

The Petroleum Equipment Institute was fully engulfed in flames Tuesday night when firefighters arrived, just hours after employees went home for the day.

Employees woke Wednesday morning, to a day of recovery, instead of work.

Employee, Chris Bouldin said, "What would have happened if this was during office hours? No one really saw it progressed, so no one saw how fast it moved."

Smoke spread next door to the Valero convenience store, where a customer got the attention of store owner Dave Hubble.

"He said, 'Is that supposed to be smoking like that?' And I go, 'Absolutely not!'" Hubble said.

Tulsa Fire Captain Stan May said as crews rushed in flames spread to the attic.

"I could see the smoke just billowing out from the roof and I knew it couldn't be good, said Hubble.

Firefighters made it out safely as the roof collapsed, taking with it years of the trade association's documents and mementos.

"We can replace the computers and the books and everything else that got burned, but we can't really replace the employee," Bouldin said.

Fire investigators were at the building through the afternoon, but no word yet on what caused the fire.