Most murderers are unaware of even the simplest clues that might give them away. But by thinking like a forensic scientist, is the "perfect" murder possible?
By Nicola Cook
The so-called perfect murder - an undetectable crime where the killer gets away scot free - has long been the stuff of mystery fiction.
From the icicle to the leg of lamb, authors have strived to come up with more imaginative weapons and crimes than their predecessors. But would any of these ingenious methods work?
Staging a death to look like an accident is one of the first things an aspiring murderer might try. But when a pathologist examines the body of the victim they're not just looking for the cause of death.
Other minor injuries - like bruising from a struggle, or scratch marks where a body has been dragged - can reveal a murder.
Another important clue is hypostasis. When someone dies their blood pressure falls, and gravity makes the blood pool in those regions of the body closest to the ground. This red staining is clearly visible, and can reveal the position of the body at death.
If a body is found lying on its right-hand side, but the red staining due to hypostasis is down the left-hand side, then the body has clearly been moved.
"It gives us a very important clue," says pathologist Dr Richard Shepherd. "If you're dumping a body, of course you don't think where's the hypostasis, you just dump the body and run."
The body can reveal a lot of secrets, so a number of killers have tried to get rid of their victim's body altogether.
In the 1940s the "acid bath murderer" John Haigh attempted to dissolve the bodies of six victims in sulphuric acid. This destroys bones and tissue but it can't digest fats - or plastic.
This is how John Haigh was caught. A pair of dentures, a red plastic handbag, and three human gallstones - covered in a layer of fat - were retrieved from the acid sludge found at his workshop.
One mistake and a killer is caught
Another method of body disposal comes from forensic scientists themselves.
DNA expert Eleanor Graham at the University of Leicester says: "It was developed for situations like the mass graves in Bosnia Herzegovina, where you find mostly skeletonised remains, but a bit of soft tissue adhering.
"You need to clean these bones up and to preserve the DNA for identification purposes. The old systems included boiling at high temperatures which was extremely destructive to the DNA."
What these scientists now do is simply heat the remains gently with biological washing powder. The biological enzymes eat away at the flesh, leaving just the bones behind.
This would certainly make a murder more difficult to investigate but not impossible.
Gunshot and knife wounds may still be visible on the bones, and any blow with a blunt instrument is likely to have shattered or cracked them.
But there is a weapon that wouldn't leave visible evidence behind. It's the one professional assassins favour - poison.
The victim may not even realise what's happened until the murderer is many miles away.
The most recent example is Alexander Litvinenko's poisoning with polonium late last year. It took the investigators weeks to identify the poison, because polonium-210 had never been used in this way before.
But what Litvinenko's poisoner may not have realised is that this ingenious poison might also give them away.
The alpha radiation given out by polonium-210 can be detected in incredibly small quantities. As the poisoner escaped they were leaving behind a radioactive trail that may yet lead to their capture.
This type of trace evidence is the final piece of the perfect murder puzzle. Modern crime scene investigators can spot much more than just fingerprints.
The CSI team can pick up tiny fibres of clothing, shoeprints from a carpet, and of course DNA.
The world's best detectives are one step ahead
And Ms Graham says our DNA may actually make some of us better criminals than others.
"Some people are more likely to deposit their DNA than other people. One person could pick up a bottle of water, put it down, you get a full profile.
"Another one could drink from it and you wouldn't pick up very much. No one knows exactly why this is but there does seem to be a very big difference."
So if someone sheds less DNA, that would reduce the risk of leaving tell-tale evidence.
But that's also why the real-life "perfect" murder is likely to fail. It relies too heavily on chance. Every single element must come together perfectly.
The forensic scientists need just one mistake and the criminal could be caught.
If someone wants to get away with murder their best hope is to be very, very lucky.
Horizon: How to commit the perfect murder is on BBC Two at 2100 BST on Tuesday 8 May
A selection of your comments appears below.
One of the American Mafia bosses had sign above his desk saying something like "the best secret is known by three men, and two of them are dead". The only way to commit a "perfect" murder is by conspiracy and to have bent cops on your side....there is always a mistake, or a link to the killer otherwise. The other way to do a perfect murder would be in a totally random way-like some kind of psycho, killing a stranger then leaving the country.
Phil Harrington, Newport S Wales
If you want to kill someone and get away with it; run them over. Many car-related crimes are unsolved and killing someone with a car is seen, in our society, as far less serious than doing so with a gun. Has anyone found the white Fiat that was in the underpass with Diana's Merc?
The perfect murder was the execution of Charles I. The man had a mask on, there were no forensics then, and despite scores of witnesses, the killer was never even questioned.
Nigel Macarthur, London, England
Of course there may be thousands of perfect murders a year - the point being that if they are perfect murders then no-one will recognise them as such, they will be suicides, accidents or natural deaths. We therefore have no way of knowing quite how good current forensics are.
Perfect Murder? Elvis Presley.
Perfect murders happen all the time. Every unsolved murder, every missing person, perhaps. The greatest perfect murders have probably already happened because the police defined them suicides or accidents. So the perfect murder is one that one even thinks is a murder.
Reno, Southampton, UK
If people think that shows like Quincy or CSI show how to commit a perfect murder then they are very much mistaken. Both shows are riddled with scientific errors even an A level student would see through. Regardless of forensic evidence, it's still possible to be convicted of murder even without a body being found.
John Airey, Peterborough, UK
I think that there is the "perfect" murder, but probably just by accident. JonBenet Ramsey is one example.
Kara Tyson, Mobile, AL USA
This is all very interesting, but is it really responsible journalism? Programs like CSI have already been accused of the "CSI effect" - and they're really just entertainment programs, so the details aren't always realistic or accurate. Even if people don't use your information to get away with crimes, they may think they'll be able to get away with crimes. Therefore, people might commit crimes they'd otherwise have not committed, because of fear of getting caught. There is some information that should be closely guarded - here, the benefit of the information is limited, whilst the potential cost is high.
Ankush, Leeds, UK
Ankush, get a grip. This article reveals very little, and probably nothing that couldn't be found in an introductory pathology text book.
I think that this article is irresponsible. Isn't there enough crime and murders in the world without giving hints on how to create the perfect crime and avoid being caught?! Some people may read this article and find it interesting. However, what about the people who aren't of sound mind? Surely this puts ideas into their head.
Victoria Charles, Leeds
While I see merit in comments relating to copy-cat style happenings, if someone is willing to commit a murder, they will do it regardless of whether they read/saw something that would 'help' them. There is way too much finger-pointing going on in today's society, what is wrong with portraying an informative subject on a darker matter? It's much better than another show about driving decorated cars too fast or making way for another fickle reality show.
Simon Clements-Hawes, Truro, Cornwall
Sounds like an interesting programme - pity it clashes with CSI.