Federal workplace safety officials have launched an investigation of the illnesses of three workers last week at the DuPont Co. chemical plant in Belle."We are currently investigating," said Leni Uddyback-Fortson, a spokeswoman for the U.S. Occupational Safety and Health Administration.Last week, on Monday evening and Tuesday morning, two DuPont employees and one contractor were sent to the hospital after they felt ill while working in the same area of the Belle facility.One of the DuPont employees remained hospitalized from Monday night until Friday morning, but the company has not disclosed any information about the specific symptoms and only limited details about the incident.DuPont spokesman David Hastings said in a prepared statement that the workers were performing routine maintenance in an area of the plant that had been closed down for scheduled maintenance. DuPont officials believe they may have been exposed to some sort of sulfur compound released from a process waste residue, Hastings said."We investigate all incidents to understand root cause so that we can prevent recurrence," Hastings said. "Once the investigation is complete and we have identified the root cause, we will learn from it and take the appropriate measures to ensure that it does not happen again. We are committed to operating this facility safely."The DuPont plant's safety practices have been under increasing scrutiny since January 2010, following a series of incidents that included a phosgene leak that killed a longtime plant worker.DuPont paid $43,000 in fines for OSHA citations following those incidents and had paid another $6,200 in fines to the agency following two other inspections last year. Prior to the January 2010 incidents, OSHA had last inspected the Belle plant in March 2005.After its own investigation of the Belle plant incidents, the U.S. Chemical Safety Board urged DuPont to re-examine its corporate safety practices and policies. Board investigators had found common threads -- including poor maintenance practices, ineffective warning alarms, and insufficient accident investigations -- among the three incidents that occurred over a 33-hour period.Reach Ken Ward Jr. at firstname.lastname@example.org or 304-348-1702.