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PHILADELPHIA (AP) — A woman and her three children were found dead after a house fire in Philadelphia, authorities said.

Crews were called to the blaze at the row home in the Kensington neighborhood shortly before 8 a.m. Saturday and found heavy flames and smoke coming from the first and second floors of the row home, Fire Commissioner Adam Thiel said.

Sixty firefighters, medics and support workers responded to what he called an “aggressive interior attack” as crews pushed hoses into the building, he said.

A 35-year-old woman was found downstairs, and her three children — 9-year-old and 17-year-old males and an 11-year-old female — were found upstairs, police said. All were pronounced dead at the scene by a medic unit just after 8 a.m. Saturday.

Javon Davis, assistant deputy fire commissioner, said the city medical examiner will determine whether the deaths were fire-related.

Thiel called it “a very difficult morning for us.”

“The worst thing for any firefighter is not being able to save a life,” he said.

The cause of the blaze remains under investigation.