Nearly half of the Arco Chemical Co. plant employees were returning to work today but the plant remained out of operation as investigators tried to determine what caused an explosion that killed 17 workers last week. About 150 of the plant's 350 workers were told to return to work, said Gerald Davis, Arco Chemical spokesman. ``We'll just bring some administrators in,'' Davis said. ``Other employees will be out until a judgment is made about the safety at the plant and we will continue to pay their salaries and try to keep them informed about what's happening in the plant.'' The sprawling plant complex will be brought back into operation in phases that could span several months, Davis said. Company officials and representatives from the Occupational Safety and Health Administration toured the crumbled remains of a block-long area of the 564-acre chemical plant on Sunday. Seventeen workers from Arco and contractors Austin Industrial Inc. and Waste Processers Industries Inc. were killed Thursday night when a wastewater tank exploded in a fireball. Investigators have been unable to determine the cause of the blast. The widow of a millwright killed Thursday said her husband called her a half hour before the plant blew up, saying workers restarting equipment there were afraid it might explode. Lori Davis said her husband, Michael Davis, said they were afraid to restart a compressor that they had fixed. ``They thought it might blow,'' Mrs. Davis said. Harold Sorgenti, president and chief executive officer of Arco Chemical, earlier said workers were repairing an electric compressor near a 900,000-gallon wastewater treatment tank when the explosion occurred. The tank separates hydrocarbons from water and then a compressor pumps the sometimes volatile vapors into a pipeline where impurities can be removed before releasing the gases to the atmosphere, officials said.