KINSTON, N.C. (Feb. 4)—The explosion at West Pharmaceutical Services Inc. that killed four workers and injured dozens of others was an accident, according to federal agents. However, investigators still don't know what caused the Jan 26 blast, "The Charlotte Observer" reported. U.S. Chemical Safety and Hazards Board members continue to say that rubber dust created by West's pharmaceutical product manufacturing likely caused the explosion. The newspaper report said that on Monday investigators identified several potential sources of explosive dust and studied whether or not a low ceiling in the processing area limited ventilation and caused dust accumulation. West company officials said the factory had dust-suppression systems in place. After a five-day investigation, federal Alcohol, Tobacco and Firearms agents said they have completed their investigation. "The conclusion is the explosion was an accident," an ATF spokesman said. "There was nothing to indicate there was any kind of criminal activity."