Explosion in France: What is the Cause?

Radio "France Bleu" reported today that "one person was killed and others were injured in the town of Lamoivi in northern France, in a powerful explosion that could be heard several kilometers away." The radio noted that "five people gathered at the home of a deceased individual to sort through belongings. After that, in violation of the rules, they ignited a fire outside to burn unnecessary items. After adding a bag containing an unknown substance to the fire, a strong explosion occurred that was heard seven kilometers from the fire's center." It added: "A 64-year-old man died, and another man of similar age was transferred by air to the hospital with serious injuries. The explosion shattered windows and demolished roofs of nearby buildings. Twenty-five firefighters and other emergency services arrived at the scene."