(POTTSTOWN, Pa.) — At least four people are dead after a house explosion in Pottstown, Pennsylvania, officials said.

Pottstown Borough Manager Justin Keller confirmed the fatalities during a press conference and said two other people might still be missing. Two others were hospitalized, and the extent of their injuries is unclear.

It was not immediately known what caused the explosion.

The blast happened just after 8 p.m. on North Washington Street.

Resident Christian Gonzalez told ABC News Philadelphia station WPVI he initially thought the explosion was thunder.

“I really thought it was really loud thunder just nearby, but it shook the area,” he said.

Police have not identified the victims yet.

Pottstown is located about 40 miles northwest of Philadelphia.

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