(CNN) One person was treated for smoke inhalation after a Union Pacific train derailed and caught fire on a bridge over Tempe Town Lake in Arizona, officials said Wednesday.
Plumes of black smoke could be seen Wednesday morning rising into the sky beneath a trail of flames on video and images from the site of the derailment, where part of the bridge collapsed.
The National Transportation Safety Board said via Twitter that it would investigate but that it was not traveling to the crash site at this time.
The fire reached four alarms, which Tempe Fire Chief Greg Ruiz said was a "very, very large event."
"The focus of the initial operation was to contain the fire and keep it from spreading to additional cars," Ruiz told reporters. "The fire has been knocked down and the crews continue to address any hotspots along the bridge."
Now, first responders were focused on containing leak of hazardous material from one of the cars, he said.
Tempe police Chief Sylvia Moir urged residents via Twitter to stay away from the scene, which she described as very dangerous.
"There's fire pouring into the lake from the middle of the bridge," Camille Kimball, who was riding her bike under the structure, told CNN affiliate KTVK/KPHO. "It looks like a scene from hell, truly."
The freight train, headed to Phoenix from Tucson, derailed about 6:15 a.m. local time, Union Pacific spokesman Tim McMahan said in a statement.
Eight to 10 train cars were on fire, according to McMahan.
The south side of the bridge collapsed, sending rail cars into an empty park below, the statement said. The bridge had its annual inspection on July 9.
One person was treated for smoke inhalation, McMahan said. The train crew was not injured.
Three tank cars were on the ground under the bridge, the spokesman said. Two contained cyclohexanone, a flammable and colorless oily liquid used to make nylon and as a solvent. The other carried a rubber material.
McMahan said the tank cars were not reported leaking, and not involved in the fire.
But Ruiz said the liquid had leaked from one derailed car onto a "drybed area."
State environmental officials were monitoring air quality and found "no imminent hazard," according to Ruiz. He said authorities had responded to a smaller derailment in the same area on June 26 -- which resulted in a fire on some rail ties.
A firefighter was treated for dehydration, Ruiz said.
Fire department spokesman Brandon Siebert had earlier said officials were "erring on the side of caution" and keeping firefighters and bystanders "as far away as possible" until they know more about the hazardous material on the train.
The operator was safe and assisting hazmat technicians, according to Siebert.
The cause of the derailment is under investigation. Investigators with the Federal Railroad Administration were en route to the crash scene, KTVK/KPHO reported.
"It is possible the derailment led to the partial bridge collapse but don't know that for sure at this time," Siebert said.
Nearly 100 firefighters from Phoenix, Tempe, Chandler, Mesa and Scottsdale responded to the scene, according to Siebert.