A vacant pizza restaurant exploded on Saturday in a thundering roar at a south Florida shopping plaza, injuring more than 20 people as large chunks of concrete flew through the air.

The blast flung debris widely along a busy road in Plantation, west of Fort Lauderdale. The restaurant was destroyed, and nearby businesses and cars were damaged.

Though firefighters found ruptured gas lines afterward, authorities said it was too early to determine a cause.

“We thought it was thunder at first, and then we felt the building shake and things started falling. I looked outside and it was almost like the world was ending,” said Alex Carver, a worker at a deli across the street from the explosion. “It was nuts, man. It was crazy.”

The explosion scattered large pieces of concrete as far as 50 yards and sent pieces of metal as far as 100 yards across the street. Carver said two of his co-workers’ cars were destroyed.

At least 21 people were injured though none of the injuries was life-threatening, police sergeant Jesica Ryan said.

The explosion demolished the building, leaving behind only part of its metal frame. The restaurant, called Pizzafire, had been out of business for several months. The blast also blew out the windows of a fitness club next door at the shopping plaza in Broward county.

Jesse Walaschek had just left the fitness club with his wife and three children, aged four, six and eight. They had parked near the restaurant and had driven about 50 yards away when they heard the blast.

“It was a massive explosion like I have never experienced,” he said, saying dust and debris filled the air.

“Everything just stopped. You didn’t see anybody. I just wanted to get these guys safe,” he said, pointing to his children. “If this had happened a minute before when we were getting the kids in the car, it would have been really bad.”

Dozens of firefighters responded and could be seen picking through the rubble with dogs sniffing through the debris to make sure people weren’t trapped underneath. There were no known fatalities immediately after the explosion.

Fire chief Joel Gordon said there were ruptured gas lines when firefighters arrived, but he couldn’t say for certain that it was a gas explosion.

“At this point, nobody was killed. Thank goodness for that. As bad as it is, it could have been a lot worse,” Gordon said.