Firefighters are among seven people injured after an apartment building partially collapsed following an explosion in south Dallas. The firefighters were responding to a natural gas leak at the two-story apartment complex, Dallas Fire-Rescue said in a statement. When firefighters arrived around 10:30 am local time, they noticed an odor of gas around the building.

Flames and heavy smoke had engulfed the building by 11 am, Dallas Fire-Rescue spokesman Jason Evans told reporters in a news conference Wednesday afternoon. Half of the building had been badly damaged and piles of debris from the building could be seen strewn around the structure, ABC News reported.

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“The damage is extensive,” Evans said. “The collapse itself is extensive.”

The injured, two of whom are critical, were taken to local hospitals, officials said.

Residents began reporting the smell of gas as early as Tuesday night, Evans said. Officials are still probing the exact cause of the explosion.

On September 12, four people were injured after an explosion caused an apartment building in suburban Atlanta to partially collapse. A local utility had received a call from a resident about a strong odor of gas from the three-story structure. Two people who were unaccounted for and feared to be trapped in one of three apartments that had completely collapsed, were eventually found safe.

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In August, four people were injured after after the front of a supermarket collapsed in Las Vegas. Around 20 to 25 people were reported to be in the building at the time of the incident. All utilities to the supermarket were turned off following the collapse. “They’re very fortunate. Later in the day, it might have been a different story,” said Clark County Fire Department Battalion Chief Steve Broadwell.

Broadwell said the collapse had been caused due to undetermined structural deficiency.