A Suffolk prosecutor today said a U.S. Marine recruiter is a "person of interest" in the disappearance of a 21-year-old Boston recruit. Frank Walter Lipka, 39, is being investigated by Boston police for the disappearance of his roommate, Joseph Brancato, 21, prosecutor Michael Lefleur said. Brancato, a Marine recruit, reportedly was last seen in November.Lipka was held on $10,000 cash bail today on an unrelated assault with a dangerous weapon charge.