Investigators were preparing to seek an arrest warrant for a defrocked Roman Catholic priest long considered a suspect in the 1972 killing of a western Massachusetts altar boy shortly before his death last week, a prosecutor said Monday. Incriminating admissions Richard Lavigne made in a series of recent interviews while in a medical facility further implicated the longtime suspect in 13-year-old Danny Croteau's death, Hampden District Attorney Anthony Gulluni told reporters. Gulluni said his office was prepared to prosecute Lavigne for Croteau's killing, but he died Friday at the age of 80 before he could be arrested.