Not Proven

The investigators put the evidence together, find out who did it and then realise that the perp is going to get off scot-free because the evidence won't convict them. This typically involves something being excluded from the trial, so that the audience is in no doubt about the suspect's guilt.

This entry is named after the third possible verdict in the Scottish legal system, which the legal system treats as "innocent" and public opinion typically treats as "guilty."

The "Scots verdict" is sometimes translated as "Not guilty and don't do it again", which is why some Scots lawyers want rid of it and the stigma it can attach to clients. Oddly enough, it is actually the "not guilty" verdict which is Scotland's Johnny-come-lately; Scottish juries were once asked merely to determine whether the case against the defendant was proven or not, until one jury in 1728 sought to make it clear that they thought the poor sod on trial innocent, even though the case had technically been proven.

Note that in American law, "Not guilty" means "not proven". It does not mean innocent, whether or not the defendant really is innocent. It's up to the government to prove someone broke the law. If they can't, they go free. This tends to lead to Convicted by Public Opinion in higher profile cases, where unless a new perp is brought up people (and the media) will tend to assume a defendant only went free because the courts were unable to prove they did it. Legally, not guilty is considered innocent, although you can be sued in civil court and the government can still seize your property even if you are found not guilty. They can also sometimes charge you with "civil rights" violations, as well. Isn't the law fun?

Compare Conviction by Contradiction, where the detectives appear to erroneously believe the evidence is enough for a conviction.

Comic Books

 * In the Gotham Central comics series, Dirty Cop and cop killer walks free because he was able to call in a favour from equally dirty colleagues so they'd swap the evidence; thus while the good cops could link him to the gun, they couldn't prove the gun was the murder weapon.

Fan Fiction

 * Sirius Black became an example of this trope in "My Parents' Secret Keeper". He got acquitted but the wizarding world believed him to be guilty. Dumbledore was so afraid Sirius would go after Harry he even asked Remus Lupin to spy on him. Fortunately, while Dumbledore and Arthur Weasley (who was the Minister of Magic in that fic) were trying to persuade Lupin into spying, Percy appeared to tell Arthur about Scabbers being missing (Peter wouldn't stick around with an acquitted Sirius around) and the way Percy described Scabbers to Remus made the werewolf know the truth.
 * This was not the only fic where Sirius becomes an example of the trope. In "These Grim Bones", because Fudge was afraid Sirius might have revealed some of the muggles caught at the explosion could have been saved had the obliviators noticed them, all Sirius could do was answering questions regarding his own guilt. Namely, he told he didn't kill Pettigrew, he didn't kill those muggles and didn't betray the Potters to Voldemort. Despite the fact Sirius stated it while under Veritaserum, Dumbledore still believed Sirius to be guilty and, being unable to prevent Sirius' acquittal, persuaded Fudge into sending a Hit Wizard or a Hit Wizard to (officially) protect Sirius from people who might decide to take justice on their own hands but (extra-officially) find and report evidence to send Sirius back to Azkaban. Fortunately, the witch who took the job was Amelia Bones, who did it so the job wouldn't be taken by someone who, hoping for a quick promotion, would forge evidence. She believes Sirius to be guilty but she's too honest to approve convicting anyone (even a Death Eater) on false evidence.
 * In "Growing Up Black", when some relatives of Sirius Black found out he was never allowed to plead his case in a trial, his maternal Grandfather argued at the Wizengamot that Dumbledore's testimony was just hearsay and testimony from muggles was unadmissable. Then, when asked if he wanted Sirius to be given a trial, his Grandfather instead invoked a law stating that, as a pureblood, Sirius shouldn't be forced to stay in Azkaban for more than 30 days without a trial and, because he was, the charges he'd been sent there for should be dropped. People who still believe Sirius to be guilty usually state he got Off on a Technicality.
 * In "Three Dursleys, a Potter and a Black", Sirius was acquitted and Dumbledore mentioned the blood protection so Harry wouldn't be taken away from the Dursleys. Taking advantage of the fact Dumbledore never ordered him not to move into Privet Drive and that the Dursleys love money more than they hate magic, Sirius moved into their home. When Dumbledore found out Sirius reached Harry and the Dursleys despite the wards keeping them from being found by anyone meaning to harm Harry, he assumed Sirius used some dark magic to get past the wards.
 * In the Mega Crossover fancomic Roommates (for a lesser extent in its Spin-Off Girls Next Door, because there he doesn't even bother covering up) Jareth's (and by extension his family's) involvement in any shenanigans and pranks ever can be summed up with this.

Literature

 * Invoked literally in Anne Perry's The Sins of the Wolf.
 * The Lord Peter Wimsey novel Have His Carcase ends with Peter and Harriet knowing who committed the murder, how it was done, and that there's no possible way he could ever be convicted for it.
 * In Harry Potter and The Goblet of Fire, it was revealed that, when Voldemort killed his muggle relatives, the residents of the village where they lived were sure the victims' gardener, Frank Bryce, was the murderer, despite the muggle authorities having to release Bryce for being unable to prove the victims were murdered in the first place.

Live Action TV

 * CSI quite a bit.
 * An episode of CSI: Miami rests on avoiding this. The CSI have evidence linking a person to a drive-by, but the key evidence is a testimony from the victim's son, who is a child and whom no one wants to put in front of a jury. The CSI race to find more evidence to make up for the kid..
 * Law and Order Special Victims Unit does this a bit too, although usually the perp gets off from something the main characters think should be illegal, but actually isn't (see There Should Be a Law for examples).
 * In the Law and Order UK episode "Samaritan" (based on the Law and Order episode "Manhood"), the team discover a homophobic policeman waited for four minutes to assist his gay partner who was caught in a crossfire and died- the gunman was also killed). The CPS decide that they're never going to make manslaughter by gross negligence stick and instead go for misconduct in a public office. Unfortunately,
 * NCIS had one case where the perp was a gang leader, and the team knew he had killed a sailor and the previous leader, but couldn't prove it.
 * Cold Case had one episode end with this, in which a  admits to Valens, off the record, that he committed the murder years ago. Unfortunately, , in a misguided show of loyalty , has already confessed to everything, and there's no evidence to contradict her claims.
 * Justified plays this straight and subverts it when the kidnapping and attempted murder charges against Boyd Crowder are dropped. The testimony of the only witnesses, Ava Crowder and Raylan Givens, is deemed tainted and can be used to easily create reasonable doubt in a jury. Boyd did intimidate Ava into staying in the house till Raylan arrived so the kidnapping charge is valid (though difficult to prove), however, Boyd drew his gun in response to Ava pointing a shotgun at him so the attempted murder is primarily (it is implied that he was still deciding whether to try and kill them) a case of self defense.
 * The reason the prosecutors rely on the kidnapping and attempted murder charges so much is because they have no evidence to convict Boyd of the charges he is 100% guilty of: bank robbery, blowing up a church and murder.
 * Boyd's father Bowden is convicted of the charges he is guilty of but the sheriff who arrested him is found to be working for a Columbian drug cartel so a retrial is ordered. The prosecutor decides not to bother proving the case again since Bowden only has a few months of his sentence left anyway.
 * In season two, murder charges against  are dropped after the only witness recants his testimony. The case would still have been difficult to prove since the witness was an accomplice and the physical evidence is more incriminating toward him than  . Everyone knows what actually happened but an unbiased jury would have probably acquitted barring more incriminating evidence.
 * An episode of Criminal Minds has the team arrest a man who fits the profile perfectly, and who has no alibis for any of the murders, but as there isn't any definitive evidence to say he did it, the team aren't sure what to do with him. After he is released, the team enact a plan to get him to confess, but.
 * Baltar's trial in the rebooted Battlestar Galactica ends with a not-guilty verdict that Adama (who was the swing vote on the judges' panel) explains this way, saying to Roslin that "not guilty is not the same as innocent" and that the verdict is because "the defense made their case; the prosecution didn't". A good decision, considering that Baltar happened to be innocent of the specific charges brought against him

Video Games

 * In the second Laura Bow game, even if the player has figured it out already, the murderer will get off scot-free if you haven't collected enough evidence to prove it. In the first game, the murderer regardless of what you do, collect, or figure out.
 * Most Ace Attorney cases seem like they'll end like this, until Phoenix/Apollo/Miles puts everything together at the last second. There are a few exeptions:
 * The case from the third game.
 * The final case of Apollo Justice: Ace Attorney revolves around this: the murderer has covered his tracks well enough to leave no direct evidence linking himself to the crime, but all the other facts at hand point quite definitively at him. The problem is, the existing court system in place in the series' universe requires either evidence, a confession, or incredibly strong testimony...which is why Phoenix has spent the past seven years working towards the re-establishment of a jury system.
 * Happened in Ace Attorney Investigations with Manny Coachen.

Western Animation

 * When Pepper first met Tony in Iron Man: Armored Adventures, she told them that, after the plane crash where Tony's father disappeared, the FBI spent a whole year investigating Obadiah Stane as a suspect and dropped the case since they found no proof of either guilt or innocence. Tony spent the first season believing Stane was guilty until.