OKLAHOMA CITY - People trying to get in a morning workout were forced to evacuate an Oklahoma City fitness center when a fire threatened the building.

Just after 6 a.m., firefighters were called to Lighthouse Sports and Fitness at 3333 W. Hefner Road. A fire had started in some construction materials outside gymnasium. The fire then spread to the building, filling it with smoke.

The building was evacuated. Because of all the smoke, crews called a third alarm on the fire.

Firefighters arrived within minutes, and contained and doused the flames. No one was hurt.

Investigators are trying to determine what started the fire. It's not known yet how much damage it caused.