Aside from scrawled newspaper sketches and one ageing death mask, few clues exist as to the true appearance of the infamous Burke and Hare.
Now, Scottish artist Graham McKean has started work on creating what he believes will be the most accurate "posthumous photofit" of the deadly duo.
He has been commissioned by a mystery collector to produce the pictures of the West Port murderers, whose reviled acts made them notorious hate figures of the 1820s.
It comes as a new film chronicling the killings – starring Simon Pegg and Andy Serkis – is due for release later this year.
Mr McKean said: "There's a notoriety associated with serial killers and a fascination with people who are capable of that kind of evil and that's what I want to try to portray."
Sixteen people died at the hands of Burke and Hare, with the cadavers famously sold to Dr Robert Knox at his laboratory in Surgeon's Square.
Burke was hanged in public while his accomplice William Hare, who had turned King's witness, fled Edinburgh after escaping the hangman's noose.
For the last three weeks the artist has pored over primary source material, including newspaper sketches, press descriptions and Burke's infamous death mask, to identify key physical characteristics of the men. He has already produced two sketches to work from.
"It's a fact there was at least one death mask made and some drawings that made it possible to see what they would have looked like as people.
"It's my intention to paint the most accurate portraits of the pair that has been done to date."
Mr McKean is also taking inspiration from a live mask believed to have been cast by William Hare, found mothballed in a store cupboard at Inveraray jail, Argyll, in February 2009, though its authenticity has been disputed.
Chris Henry, curator of the Royal College of Surgeons Museum which houses a Burke and Hare exhibition, said:
"There are some newspaper sketches from the time that are really not that great," he said.
"You could just about tell that Hare was much thinner than his accomplice Burke. But if you compare Burke's death mask we have in Surgeons' Hall with the press sketches there is a huge difference, so it will be interesting to see how it turns out."
Each portrait, painted with oil, will measure 12 inches by eight inches, and they are expected to be completed by the weekend.