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. Last Updated: 07/27/2016

Die in French Cave

ROUEN, France () -- Nine people died as a result of an outing to a cave in northern France that German troops once used during World War II to conceal V1 "doodlebug" rockets, rescue workers said Thursday.

The dead included three teenagers -- whose attempts to explore the cave led to a fatal rescue mission. The father of one of them and five members of the search team also died.

The nine were apparently killed by a high concentration of carbon monoxide gas in the tunnels, outside the village of Buchy near Rouen. The lethal gas may have been created by a fire lit by the teenagers.