Investigators in Cedar Rapids have determined part of the cause of a fire and explosion that heavily damaged a fast-food restaurant. The explosion April 24th blew out the dining room windows and front door of a Burger King restaurant on the southeast side of Cedar Rapids.

Firefighters were just getting ready to enter the restaurant to deal with the fire when the explosion happened. One firefighter was knocked backward by the explosion, but was not injured.

A report from the fire department says the fire was started unintentionally by an employee who had attempted to put a cigarette out on the wall outside the building.

The fire spread from the outside wall and led to the explosion. The investigation though showed no damage to the natural gas piping or equipment failure that could have led to the explosion — and it’s not known if the explosion was caused by natural gas or sewer gas.

The insurance company that covers the building is conducting its own independent investigation, but has not submitted a report to the fire department.