Detectives investigating a robbery at Latchingdon Post Office are urging residents to check their bins for a mask or bat.
Officers would like residents living in the area to check their gardens, bins or hedges for any unusual items that may have been discarded.
Detective Inspector Neil Pudney, of Chelmsford CID, said: "We are looking for a discarded Halloween style mask, white with black eyes, or bat that may have been used in the robbery.
"I would urge anybody who finds these items to contact officers at Chelmsford CID immediately. "
The robbery happened at about 3pm on Thursday at the store in The Street in Latchingdon.
A man armed with a bat and his face covered by a mask, entered the post office and threatened the cashier, a woman, who escaped out of the back of the post office.
The man is described as 5ft 7-9 inches tall, stockyor well-built and was wearing a bomber type tan jacket. He stole a four figure sum of money and ran out of the store.
Anyone with information is urged to contact detectives at Chelmsford on Essex Police 101 or Crimestoppers anonymously on 0800 555 111.