A picture taken on October 3, 2017 in Paris shows the building where French police discovered a homemade bomb the day before. The Paris prosecutor has opened an anti-terror investigation after the discoveryand five people have been arrested, sources close to the case said Monday. Police discovered two cylinders of gas in the hall of a building in the city's 16th arrondissement and two others on the sidewalk. They also found a mobile phone linked to the cylinders, which appeared to be "a device to ignite" the explosive, the sources said. / AFP PHOTO / Martin BUREAU