MEXICO CITY (AP) — Police in Piedras Negras say that an apparently accidental explosion in a customs building on Mexico’s border with the U.S. has killed one woman and injured five other people.

Felix Alberto Lopez Arenas is the director of operations for the local municipal police in the city on the border with Texas. He tells The Associated Press that the woman who died in the Sunday blast suffered a fractured skull and the other five people had burns.

Lopez Arenas says the blast occurred in the morning in a dormitory for customs agents. The cause of the explosion is under investigation, but authorities believe it was caused by an accumulation of gas.

The explosion did not affect border operations.

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