A nationwide manhunt is under way looking for the gunman believed to have killed a Cotten Apartment resident in his home Friday night.The suspected murderer, Gregorio Arenas, is considered armed and dangerous and is wanted on criminal homicide charges.The victim, Albertano Guerro Trejo, 29, was shot dead in his Cotten Apartment residence off Cascade Avenue around 9:30 p.m. Friday. He was killed by a single shot to the chest from a 9mm handgun allegedly wielded by Arenas. Police found the victim dead in the floor of his home when they arrived to investigate the call of a shot fired.