JACKSONVILLE, Fla. -

Dozens of firefighters battled an explosion and fire at an apartment building under construction Tuesday afternoon near the St. Johns Town Center.

Jacksonville Fire-Rescue called a second alarm to get additional units to battle the blaze on the five-story building. In all, 60 firefighters responded and put the fire out in about 15 minutes.

IMAGES:  Fire at construction site

Channel 4 meteorologist Richard Nunn said a relatively small fire broke out just before 4 p.m., but a few minutes later there was an explosion.

"There was a rumble on the ground and you could feel a concussion on your chest, then a fireball took off," Nunn said.

Channel 4 quickly received messages and tweets from people at the Town Center and motorists passing on J. Turner Butler Boulevard that saw or felt the blast and fire.

"I was driving on JTB and I saw a helicopter," witness Adam Mobley said. "When I first got here, there was two big explosions and fire was coming out the top of the roof. It was pretty intense and scary."

There were no reports of injuries despite initial reports that there were workers trapped the roof when the fire broke out. Firefighters said the workers had made it to the ground on their own by the time the first engines arrived.

Jacksonville Fire-Rescue spokesman Tom Francis said it appears two 25-pound propane cylinders exploded during the course of the fire, which grew to consume scaffolding set up around the structure.

Francis said the fire started on the roof of the complex, where workers used heating equipment to lay foam flooring down. The flooring was smoldering about 30 minutes after workers left it, and once they were downstairs, the propane tanks exploded.

Samantha Church was eating a late lunch at Brio Tuscan Grille nearby when the explosion happened.

"You could feel the entire restaurant vibrate. It was insane. It felt like a huge bomb went off or something," Church said.

The apartment complex is being built next to Seasons 52, along JTB. Police blocked off many of the roads around the area due to emergency vehicles in the area.

The State Fire Marshal said the cause of the fire was accidental, but did not release any other details. Damage was estimated to be about $80,000.

The Jacksonville Sheriff's Office said it received so many calls to 911 about the fire that some callers may have had to wait unusually long to speak to a dispatcher.

"We apologize for any inconvenience and want to thank the hundreds of callers that responded to this emergency, all within a few minutes of each other," the Sheriff's Office said in a statement.

Duval County emergency communications center receives high volume of calls

The Jacksonville Sheriff's Office Tuesday released the following statement to Channel 4 about the large number of citizens trying to report the fire:

"Due to the volume of calls our 9-1-1 emergency communications center received today, by citizens viewing the fire off J. Turner Butler Blvd. at the Town Center construction site, some callers may have experienced unusually long wait times. We apologize for any inconvenience, and want to thank the hundreds of callers that responded to this emergency, all within a few minutes of each other."

Channel 4 crime and safety expert Ken Jefferson says the experience might help the county become more prepared for future emergencies.

"When you have something of this magnitude, of course people want to become involved and of course let law enforcement officials know what's going on and what's happening. Unfortunately, they all did it at one time. They were constantly calling, which caused an over-extended period of waiting on behalf of the caller. But this is a good lesson for the sheriff's office so that they can work on ways to improve their system," said Jefferson.

Jefferson said the fire could have ended much worse "if this was a fully involved fire that had people who needed to be rescued... and possible mass casualties and lives. Certainly, the first few calls coming in would've dispatched the emergency officials there, but that will not be the only emergency going on on that particular day where someone needs the police, so there's got to be a way they can work out, come up with a way that they can figure out how they can manage this vast number of calls at one time," said Jefferson.

Before calling 911, Jefferson suggests people determine whether emergency crews are already responding.

"Common sense should tell you that if you see emergency crews there on the scene, you don't need to dial 911 because they were aware of it and that crew will call for additional units if necessary if they need it, so you don't have to constantly call 911," said Jefferson. "I understand the conscience of the people when something like this happens on this magnitude. They want to get involved and make sure the law enforcement officials are aware of it as well as firefighters are aware of it, so they did what they were compelled to do, which is the right thing, but just need to know that if you call and you either get a busy signal or you are holding for an extended period of time, they probably already have the call."