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PLANTATION, Fla. (Reuters) - An explosion obliterated a vacant restaurant at a shopping center in South Florida on Saturday, injuring at least 21 people and showering the parking lot with debris.

Firefighters found a ruptured natural gas line, but authorities said the cause of the explosion in Plantation, Florida, about six miles west of Fort Lauderdale, was still under investigation.

Video posted to Twitter showed the force of the blast blew out several windows at a nearby L.A. Fitness gym, sending patrons running for the exits.

Two people suffered serious injuries and were taken to the trauma center at a local hospital, Joel Gordon, the city’s deputy fire chief, said at a news conference.

“We’re relieved - the injuries are not as severe as we thought they would be,” he told reporters.

Gordon said firefighters found a ruptured gas line but could not yet confirm it had caused the explosion.

A witness, Evan Hoffman, told the Sun-Sentinel newspaper he was working out at the gym when he heard a massive noise.

“A huge, huge bang, thump, almost, explosion,” he told the newspaper. “It started shaking back and forth and the roof tiles started crashing down and the power went out.”