At least one person was killed and six injured Monday after an explosion at a facility owned by the world’s biggest chemical company, authorities in Germany said. Another six people have yet to be accounted for. BASF confirmed the explosion took place in Ludwigshafen at around 11:30 a.m. local time (5:30 a.m. ET) during work on a pipeline corridor. The exact reasons for the blast were being investigated, the company said on its Facebook page. BASF executive Uwe Liebelt said the explosion followed a fire in a pipeline, sending a plume of black smoke into the air. The Associated Press reported that the explosion happened after the company's fire service arrived at the scene. "How the explosion happened is not clear at the moment," Liebelt told reporters, according to the AP. He added that it wasn't yet known what substances were involved and didn't elaborate on the identities of the victims. Residents in parts of Ludwigshafen and Mannheim, which is located on the other side of the Rhine river, were advised to stay indoors and keep their doors and windows closed. Authorities instructed children and teenagers to remain in school, amid reports that people in the area were complaining of respiratory ailments. The AP reported that the facility where the explosion occurred is used to unload flammable liquids and liquid gas. Precise figures on the numbers of injured and missing were not immediately provided.