OKLAHOMA CITY - A bookstore fire in northwest Oklahoma City turned into a death investigation Thursday evening after firefighters found a man's body inside.

There was a lot of fuel for the fire, as the store is filled with books. When fire crews arrived at 30 Penn Books shortly after 4 p.m. Thursday, they say thick brown smoke was billowing from inside but the door was locked. The firefighters forced their way in and tripped through stacks of historic literature as they fought the flames.

“Our first pass through,” says Battalion Chief Benny Fulkerson, "you probably couldn’t see your hand in front of your face. It was extremely heavy smoke and, again, we were working in a lot of clutter.”

On the second pass through after the fire was extinguished, the crews found the man's body about 20 feet from the front door.

“We had a good response time here,” Fulkerson says. ”(We) got inside quickly, had the fire knocked out quickly, and, unfortunately, this one turned out that there was somebody inside. Smoke can overtake you very quickly.”

Late Thursday, investigators ruled the fire as intentional and the death as a homicide but did not identify the victim. Frequent customers believe the victim is likely the shop owner, whose car was parked outside during the fire. Those who know him tell News 9 the owner is a history enthusiast who seeks out special editions to please the book lovers he serves. They believe the only motive for murder would be robbery, as the store is a cash business.

Firefighters say the flames look to have sparked farther back than the register.

“They said it appeared to be the inventory in the rear of the store,” Fulkerson clarifies.

The businesses that share a roof with the bookstore did not receive significant damage. The store itself sustained about $25,000 worth of damage to the building and another $10,000 in inventory.