TULSA, Oklahoma - Fire investigators are inside downtown Tulsa's historic U-haul building working to figure out what started a fire there Sunday. People who are renting a storage unit are stopping by to find out the fate of their belongings.

But as of now, only investigators and cleanup crews are allowed inside.

The fire started sometime before 5 o'clock Sunday evening. Tulsa firefighters worked the scene for eight hours, putting the flames out and making sure it did not rekindle.

Captain Stan May says one firefighter was taken to the hospital to get checked out for dehydration and heat-related symptoms, but was treated and released.

Firefighters say it appears the fire started on third floor. They say there was fire damage to some of the third floor storage units and interior hallways.

As for the units below, firefighters says there will likely be some that have smoke and water damage. One woman says she's been renting a unit here for six months, while saving up to buy a house. Macayla McFee says almost all of her belongings are here - and almost all are replaceable except the keepsakes of her 3-year-old daughter.

"Her baby things," McFee said. "Just clothes and toys and stuff like that."

Firefighter say  it's too soon to say what caused the fire.

A U-Haul representative said they will be contacting renters whose units are affected.