Firefighters worked through the night to clear three commercial buildings in Norristown of carbon monoxide, which reached lethal levels in the properties after an underground fire caused a gas leak early Friday morning.

Crews responded to CO alarms that were going off in an AutoZone store on East Main Street, near Swede Street, around 1 a.m., officials said.

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Initially they were unable to locate the cause, but an investigation uncovered an underground electrical fire involving PECO equipment led to the leak, according to Mike O'Donnell of the Norristown Fire Department.

The CO readings were so high -- 3,800 parts per million -- that O'Donnell said they were capable of killing a person in minutes.

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"If they're inside the building, the levels we found could be lethal. They'd be dead," he said.

There was a risk of an explosion early on, but the firefighters brought in large fans to clear the gas out of the affected buildings -- 36 E. Main St., 40 E. Main St. and 48 E. Main St.

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Online records show those addresses house Pauline's Deli, the law offices of Cullen and Conwell, and a temporary employment agency, Labor Ready.

A PECO underground utility crew headed to the scene to fix the leak at its source.

No injuries were reported.