A senior investigator in the Westchester District Attorney's office died Sunday after mysteriously falling ill following a drug raid.Stuart Cohen took part in a drug raid at a home on Lytton Avenue in Hartsdale in December. Cops confiscated $6 million worth of heroin in the raid. Just days later, Cohen became ill with flu-like symptoms and fell into a coma.Medical experts were not able to determine what caused Cohen's illness. One theory by doctors is that the drugs were contaminated.Cohen leaves behind a wife and two children. He was a volunteer firefighter in Hastings and a former police officer in the village.