It is the greatest mystery in fiction - but an expert from Northern Ireland has helped reveal the secret of spotting 'whodunnit' in Agatha Christie's famous novels.
While the best-selling author's fans celebrate the 125th anniversary of her birth, experts say where the novel is set, the primary means of transport used in the book and how the victim dies are key clues to the killer.
The killer will be introduced within the first half of the book and is likely to be emotionally involved with the victim, according to research commissioned by UKTV channel Drama.
An expert panel - including Dr Dominique Jeannerod, senior research fellow at the Institute for Collaborative Research in the Humanities at Queen's University Belfast; Dr James Bernthal from the University of Exeter, and data analyst Brett Jacob - analysed a selection of Christie's mystery novels including Death On The Nile and Murder On The Orient Express.
They found that if there are a lot of land vehicles in the story, the killer is most likely female, and if there are a lot of nautical vehicles and aircraft in the story, the killer is most likely male.
If the victim is strangled, the killer is most likely male, or male with a female accomplice, and if the setting is a country house, the killer is most likely female.
Language used in the book to describe a female killer is usually more negative than when describing a male killer, experts found.
The female killers are normally found due to a domestic item, and male killers are normally found out through information or logic.
The panel found that if Hercule Poirot is the detective, and the cause of death is stabbing, the killer will be mentioned more frequently at the start of the book.
But if Miss Marple is the detective, and the motive for the murder is money or an affair, the killer will be mentioned more in the latter stages of the novel than the beginning.
The research was commissioned by Drama to celebrate its Agatha Christie Hour.
The TV channel is screening 10 of her most famous stories on weekdays at 8pm from August 3 to 14 to mark the 125th anniversary of her birth.