Update: Three firefighters are in critical condition, and one is stable. The three apartment residents who were injured are in stable condition.

A two-story apartment building partially collapsed, injuring at least three residents and four Dallas firefighters this morning.

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Dallas Fire Rescue responded to a 911 call reporting a natural gas leak at 5726 Highland Hills Road, near Bonnie View and Simpson Stuart.

From a DFR media release:

When firefighters arrived, the smell of gas was noticed around an apartment building near the gate entrance. At some point during the course of investigating the odor, an explosion occurred, causing a partial collapse of the two-story apartment building.

The explosion caused a large fire with heavy smoke, WFAA reports.

The six injured people were taken to the hospital, and their condition is unknown, according to the media release.

DFR will investigate, “along with multiple local, state and federal agencies, to determine exactly what happened.”