JACKSONVILLE, Fla. -- More than 100 firefighters and every hazardous-materials unit in Jacksonville battled what a spokesman called a "hellish inferno" that followed an explosion of a chemical company on the Northside on Wednesday afternoon.

Nearly two hours after the blast ignited the industrial facility on Faye Road, a Jacksonville Fire-Rescue spokesman confirmed that three people died and 14 people were transported to area hospitals. Just after 6 p.m., JFRD said a fourth victim had died.

Officials could not confirm whether anyone was still missing, but firefighters continued to battle hot spots and likely will all night.

"This was unlike any noise I'm sure any of us have ever heard," Kevin, who works at the port on Blount Island, told Channel 4. "It literally shook the whole building. We walked outside and saw what was reminiscent of a nuclear explosion -- that kind of cloud formation you see when a nuclear explosion goes off."

Callers to Channel 4 described a flash, an explosion and a mushroom cloud that was reported at 1:33 p.m. After the initial explosion, thick, black smoke and flames billowed from the area for 90 minutes.

Hospitals were put on mass-casualty alert. Four hospitals received patients, with Shands-Jacksonville Medical Center decontaminating the 11 victims who arrived there. A Shands trauma physician said one person was in critical condition, three were in fair condition and seven were in good condition. Most of the injuries were fractures and exposure to chemicals, doctors said.

Initially, six people were reported missing, but three of them were located, Jacksonville Fire-Rescue spokesman Tom Francis said. By evening, he said everyone was accounted for.

"The incident is so violent and the fire is still so volatile ... we have not been able to ascertain directly yet what materials are present," Francis said in announcing that a one-half mile radius was being evacuated.

That evacuation was lifted just after 4 p.m. after the fire was under control and air monitoring showed no presence of hazardous chemicals.

The explosion at T2 Labs, a chemical company that makes solvents and gasoline additives, became a three-alarm fire, with firefighters taking extra precautions because of potential hazardous conditions.

"It was a pandemonium," said Byron Evetts, an engineer working at the JEA plant who is also a trained rescue worker. "It was a large explosion.... Best we could tell, there was no structure left. It was like the entire structure had been removed."

Evetts said the explosion was so violent that steel beams and other debris were scattered everywhere.

"There was like this tremendous boom like I've never heard in my life," said Shannon Story, who works nearby. "Everything shook in the building, the power went out, light bulbs fell from ceiling. We all evacuated as fast as we could."

People in the area initially thought the explosion was at one of the JEA's two Northside generating stations. The explosion occurred about one block from St. Johns River Power Park, the coal plant jointly operated by the JEA and Florida Power and Light.

A JEA spokeswoman said the concussion knocked briefly two generating units off-line. The facilities were evacuated of all but essential personnel.

The Red Cross opened Oceanway Elementary School as a shelter. Little Temple Child Care Center was evacuated and the children and workers relocated to Oceanway Elementary. By early evening, everyone was gone and the shelter closed.

"Having toured the facility, the damage is really almost hard to comprehend," Jacksonville Mayor John Peyton said. "You see power lines have snapped overhead. All of the buildings surrounding this facility have experienced some type of (damage). Windshields have smashed, the walls are caved in."

The Occupational Safety and Health Administration as well as a team from the U.S. Chemical Safety Board were sending investigators. The Jacksonville Sheriff's Office homicide unit also launched an investigation.

Police blocked traffic on Faye Road between Alta Drive and New Berlin Road, but residents with identification or people with business in the area are being allowed through. The state Road 9A exits at Alta reopened about 6 p.m.