A fire broke out Thursday afternoon on the roof of the Cancer Institute at St. Joseph's Hospital in Tampa.

According to Tampa Fire Rescue, firefighters were sent to the hospital at 2:20 p.m. When they arrived, they found heavy smoke and flames coming from the roof of a lower-level portion of the multi-story building.

The source of the fire was identified as pallets on the lower roof, which were burning.

Crews battled the fire until it was under control at 2:59 p.m., which is when they began conducting overhaul procedures in an effort to make sure there were no hot spots that could flare up.

By 4 p.m., the fire was completely out.

Those who were passing by said they noticed the fire right away.

"It was really bad," witness Tenisha Gonzalez said. "It started as a small fire, then something sparked and just blew up real quick, and it started getting worse."

Investigators said the fire likely started when sparks from welding operations on the upper roof fell onto the construction materials on the lower roof, where they smoldered until eventually flaring up and spreading the fire.

The building was undergoing renovations at the time and was not occupied. All of the construction workers safely evacuated the site.

In addition, a building to the north that houses an outpatient clinic was evacuated as a precaution.

No injuries were reported.