WATER MILL - Detectives say they know who killed a nun in a Water Mill hit-and-run. Now, they say it's just a matter of finding him.Southampton Police Chief William Wilson says investigators know exactly who fatally struck Sister Jackie Walsh Monday night. Police have enlisted help from a number of agencies, including the U.S. Marshals Service, in hopes of tracking down the suspect.Officials say a man left Walsh to die on the side of a road in Water Mill, then ditched his damaged Volkswagen about a half-mile away. Police say they're continuing to canvass the neighborhood and interview those in the area.Wake services will be held for Walsh Friday from 2 p.m. to 8 p.m. at St. Edward Confessor Church in Syosset. She will be laid to rest Saturday morning.Police seek hit-and-run driver who killed nun in Water Mill