Modern medical science has exposed the villainy of the crocodile mummy sellers of Hawara, more than 2,000 years after they defied the edict of a Pharaoh and turned neatly bandaged bundles of rubbish into a nice little earner.

Before the reopening this month of the Egyptian Galleries at the Fitzwilliam Museum in Cambridge, curators took their animal and human mummies to the city's Addenbrooke's Hospital, as part of a £1.5m re-display of the internationally renowned collection, which dates in part back to the founding of the museum in 1816.

Analysis continues after the mummies were run through a CT scanner and other tests, but the preliminary results are startling. The two baby crocodile shaped mummies were originally sold to worshippers at the temple at Hawara, to be buried in ritual pits as an offering to the god Sobek. There was clearly a history of problems with the animal sellers: a pharaonic decree a century earlier had ordered that each mummy should contain the body of one animal.

The museum's kitten mummy did indeed hold a very small cat, and there was a sacred ibis within the spectacularly elaborate wrappings of another. The crocodiles however were spectacularly lacking in crocodile: one held a minute vertebra, the other a handful of straw, rags and mud without a scrap of any animal content at all.

The museum's single human mummy, collected by Flinders Petrie in the early 20th century, is exceptional. It comes from the Fayum, where the cultures of Ancient Greece, Rome and Egypt met, producing Egyptian-style mummies, sometimes with inscriptions in Greek, decorated with hauntingly beautiful portraits painted in encaustic wax.

Archaeologists have argued since their discovery about whether the images of men, women and children were idealised types or true portraits. Although a reconstruction of the head of one woman for the British Museum showed a close resemblance to her portrait, the Cambridge tests reveal a sadder truth.

The Fitzwilliam's mummy has the image of a dazzlingly handsome young bearded man, with a wreath of gold leaves in his dark curly hair. The tests show the body inside is a disaster, his back and neck broken after death, head crushed onto the chest, and apparently left so long before mummification that only the skin on the inside thighs remains. Work continues to try to establish his age, what killed him - and how wide the gulf was between the real man and the beautiful image.

· The Egyptian Galleries at the Fitzwilliam Museum, Cambridge, open free every day except Monday. www.fitzmuseum.cam.ac.uk