NEW YORK — Investigators seeking witnesses and suspects in the brutal slaying of a graduate honors student have been studying security videotape from the neighborhood where she spent her last night out with friends, police said Tuesday. One tape shows the victim, Imette St. Guillen, talking to a female friend at about 3 a.m. Saturday outside Pioneer, a bar in the Bowery section of Manhattan. The two can be seen parting ways, with the friend leaving by cab, police said. The friend later told police St. Guillen sounded fine when she checked on her by cell phone about 30 minutes...