LINZ, Austria (AP) -- An explosion rocked a discotheque in the western Austrian city of Linz early Saturday, and police said they found remnants of a hand grenade at the scene. Twenty people were wounded, four seriously.The explosion occurred at about 3:20 a.m. (9:20 p.m. EDT) at the X-Large Disco in Linz, about 120 miles west of Vienna, the Austria Press Agency reported. Bits of a grenade were found not far from the disc jockey's booth, the reports said.The wounded were taken to nearby hospitals for treatment of shrapnel wounds, Austrian radio said.