Found the Killer, Lost the Murderer

This trope describes situations in which a character has been seeking the culprit in a murder for a long, long time (in a television context, usually at least a season's worth of episodes). The character finally locates the murderer, only to find that it was a hired hit. They know who pulled the trigger, but they need to learn who hired the assassin.

Then, before the killer can be questioned, he's taken out of the scene: he escapes custody, or is killed trying to escape, or bites down on a Cyanide Pill, or otherwise arranges things so that the murder still goes essentially unsolved, despite the killer's discovery. Particularly common is for them to give in, say, "His Name Is" and then a sniper pops his head... sometimes right off his shoulders.

Might be expanded to any form of unsolved mystery, and indeed non-criminal mysteries as well.

A subtrope of Yank the Dog's Chain, as it allows them to tease the audience with resolution ("This week, we find the identity of Leonard's father's murderer!") while at the same time preserving the tension of the unresolved mystery ("...but not why!").

Unmarked spoilers follow.

Comic Books

 * The Pre-Crisis Batman, long after he finally tracked down the murderer of his parents, Joe Chill, discovered that Chill was actually an assassin hired by the mobster Lew Moxon who wanted revenge on Thomas Wayne for getting him arrested. Of course, Batman reopened the case to hunt his parents' true murderer down, a discovery that was especially galling considering Robin pointed out that by leaving him alive, Bruce was manipulated to be Moxon's alibi since a 10 year old could not be expected to know that a simple stick-up gone murderous was more than it seemed.

Live Action Television

 * This happens three times in Castle. The first time, Castle and Beckett find the man who murdered Beckett's mother and find that he's a hired killer, but Beckett has to shoot him in order to get Castle out of a hostage situation. The second time, Beckett manages to capture another hired killer, a sniper, who was hired by the same person(s) who ordered her mother's murder. This sniper is still alive by the end of the episode, but indicates with a stone-faced glare that he'll never inform on his clients. The third time involves a key person involved(really, really complicated) with her mother's murder- She gets to talk to him uninterrupted, and he knows who the mastermind behind the conspiracy is, but refuses to say the name anyway, saying that the mastermind is so rich and powerful that giving her his name would get her killed as certainly as if he'd shot her himself. He dies minutes later.
 * This also occurred in Monk, with the identity of Trudy's killer.
 * Continuing the trend of single-word titles, Life season one ends with the capture of the man who really killed Crews's business partner and said partner's wife and family. But the Man Behind the Man gets to him in prison, so Crews still doesn't know why they were killed.
 * This happens routinely in Burn Notice. Every time Michael spends half a season tracking someone down, they suddenly die.
 * This happens routinely in Burn Notice. Every time Michael spends half a season tracking someone down, they suddenly die.

Film

 * In Star Wars: Attack of the Clones, an assassin tries twice to kill Senator Amidala. When the Jedi capture the assassin, Jango Fett enacts this trope with great prejudice.
 * The Hydra assassin in Captain America the First Avenger does this as well after his submarine getaway doesn't exactly turn out as planned.

Video Games

 * Phoenix Wright Ace Attorney: Justice for All has a complete Inversion during case 4 in which the this trope is turned around. At first the killing seems like a straight forward murder. It however turns out

Real Life

 * Lee Harvey Oswald. Shot JFK, but was killed by Jack Ruby before he could be interrogated further. Perhaps if he had not died so soon afterwards, less people would speculate as to if he did it on behalf another person or organization, who perhaps put out a Contract On the Hitman to avoid him ratting them out to the authorities, or any of the other theories behind JFK's assassination.
 * Became Freakier Than Fiction when Ruby died before going to trial, making theorists expand their theories to include his death.