A Derby nurse who admitted taking kickbacks from a drug company that makes the powerful opioid painkiller Subsys is cooperating with federal investigators, who recently charged two drug company employees with violating kickback laws, court documents show. Documents filed earlier this year show that Heather Alfonso, a nurse formerly employed by a Derby pain clinic, requested a delay in sentencing because she was “actively cooperating in an ongoing investigation in several jurisdictions, including Connecticut,” in which arrests were expected. “Ms. Alfonso’s cooperation with both state and federal investigations is significant when qualifying her character and conduct, relative to sentencing,” her attorney said in filings in U.S. District Court in Hartford. A judge agreed to delay Alfonso’s sentencing until Sept. 13.