ROBBERS armed with a hammer threatened security guards after ramming a post office van in Widnes.
They stole a cash box from the vehicle in Ditchfield Road and drove off towards Speke.
But their raid backfired. When they opened the cash box, it emitted a purple dye which is likely to have stained their hands, clothes and the money.
DC Neal Davison said: "Thankfully no one was injured during the incident and officers have been making a number of enquiries to trace those responsible.
"We would like to speak to anyone who saw a blue Audi acting suspiciously in the Ditchfield Road area of Widnes, anyone who saw the attack on the Post Office van, or anyone in the Speke area who saw the men behind the shops or running away."
The robbery happened at 11.48am on Wednesday, February 26.
The offenders broke open the cash box behind shops in Alderwood Avenue, Speke.
They then abandoned a blue Audi in Elloway Road and made off on foot over gardens towards Critchley Road.
Anyone with information can contact DC Neal Davison on 101 quoting incident number 318 of 26 February 2014.
Alternatively information can be given anonymously by contacting Crimestoppers on 0800 555 111.