TEMPE, AZ (3TV/CBS 5) – Investigators are still working to determine how a freight train derailed on a bridge over Tempe Town Lake on Wednesday, sparking a massive fire and causing the bridge to partially collapse. The fire was pumping thick black smoke into the air. That smoke plume was visible for miles.

Tim McMahan, a spokesman for Union Pacific Railroad, said as many as 10 train cars derailed at about 6:15 a.m. The south side of the bridge then collapsed and caught fire. A spokesman said the train crew was not hurt but one person at the park was treated for smoke inhalation. The City of Tempe said a firefighter was treated and released after being overwhelmed because of the heat. 

Concerns About Hazardous Materials

Tempe officials said one of the derailed cars contained cyclohexanone, a pale and oily liquid that is toxic and flammable. It's used to make nylon and paint remover. HAZMAT crews placed a foam layer over the car to keep it cool and prevent any chemical vapors. The chemical is leaking into a storm drain that goes into the riverbed on the west side of the dam. Crews are testing the area and have stopped the 500 gallon leak. The other two cars on the train track were transporting chemicals but neither are a hazardous concern right now, the City said. It's unclear what impact the derailment will have on the wildlife in the area.

A Union Pacific spokesperson said the train had 102 rail cars, eight to ten of them caught fire. The cause of the derailment is still unknown. 

The cars that burned for hours were carrying lumber, some of which fell into the lake.

At a news conference Wednesday afternoon, Tempe's mayor said the EPA is monitoring the air quality in the area, but there is no imminent risk. According to the Arizona Department of Environmental Quality, air quality is good and there is no risk.

Video from CBS 5 Investigates producer Gilbert Zermeño, our first look at the conflagration, showed flames burning along the length of the train bridge, a portion of which had collapsed on the Tempe Beach Park side. (Click the video player below to watch.)

The call was escalated to a third alarm, which means more resources from all over the Valley were assigned to it.

"A SCENE FROM HELL"

Camille Kimball was riding her bike under the bridge just a moment before it collapsed. She said the noise of the train became deafening, and as she came off the bridge, she saw people with their phone cameras trained behind her.

"I turned around to look and got the fright of my life," she said. "Now there's fire pouring into the lake from the middle of the bridge. ... It looks like a scene from hell, truly. A scene from hell. ... The flames are intense and the sky is filled with black smoke." She described the experience as "surreal."

Several witnesses compared the scene to something you might see in a movie.

Helicopter pilot Bruce Haffner described the scene as "unbelievable." "It's such a catastrophe," he said. Haffner also said that helicopters were coming in to help fight the fire from the air and that air traffic controllers were having planes taking off from Sky Harbor International Airport move to the north, away from the huge smoke plume.

"We are directing aircraft north or south of the incident scene to minimize aircraft-noise distraction to fire crews," an FAA spokesman told Arizona's Family.

Although planes are being diverted around the incident, things at Sky Harbor are running as usual. "At this time, there are no impacts to our operations," a spokesman said.