Night of a Thousand Eyes

When a friend of Dan's who's also a P.I. is killed, his girlfriend asks him to look into it. At the same time, there's a P.I. convention going on at the hotel and some other P.I.s are getting killed. A female P.I. knows something but she's not willing to share it. When an attempt is made on her, she reveals that she and other P.I.'s were hired to solve a case that's been unsolved for years.