Who Killed the Strangler?

A famous wrestler is slain during a charity match and the killer is one of five people seated in the ""Gold Row"": the victim's sister (who has a split personality), his wrestling rival, a fashionable socialite, a ""method"" sportswriter, and a very wealthy old lady with a strong affinity for wrestling violence. The weapon? - a poisoned dart. How was the murder committed without the killer being seen? After watching a tape of the event, over and over, he's not sure if he hasn't seen footage of the actual killing. Burke ultimately traps the killer into confessing by claiming to have found the murderer's ""lip prints"" on the murder ""weapon"".