At least one person is believed to be dead after a possible gas explosion toppled a building in South Philadelphia on Thursday, officials said.

Firefighters located one person trapped under debris that they were not able to excavate, Philadelphia Fire Commissioner director Adam Thiel told reporters. That person is “most likely deceased,” Thiel said.

He added that the medical examiner will have to confirm the deaths.

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At least one other person is believed to still be in an adjacent building, but Thiel said officials are working to confirm how many others may be inside.

The fire started at 11:36 a.m. Firefighters were still actively fighting the fire at 4:45 p.m., Thiel said.

“It’s a very complex and dangerous incident,” he said.

Three row houses were completely collapsed and two others heavily compromised, Thiel added.

The fire commissioner told reporters the fire was “gas-fed,” according to a local NBC 10 report. Fire officials told the local outlet they received numerous calls reporting an explosion prior to the blaze. The cause remains unknown.