PARIS, April 20 (Reuters) - Frustrated Paris St Germain supporters painted graffiti at the club's training ground during the night after a 3-0 defeat at Caen left PSG in danger of going down for the first time, witnesses said on Sunday. 'They painted graffiti saying: 'If we go down, we'll shoot you down' and 'PSG in Ligue 2 = Riots',' one of the witnesses told Reuters. During Saturday's match, a PSG supporter invaded the pitch and started running towards goalkeeper Mickael Landreau before heading back. Media reported that after the match PSG fans tore the windscreen wipers off the team's bus. PSG lie 18th in the Ligue 1 standings and are three points adrift of Racing Lens, the first club above the drop zone, with four games to play. Three days ago, Interior Minister Michele Alliot-Marie banned a group of PSG supporters, the Boulogne Boys, for unfolding a racist banner during last month's League Cup final win over Lens.