A hooded robber armed with a kitchen knife queued in a post office before threatening staff at the counter.
The suspect, pictured in this CCTV image, waited behind two other customers at Thaxted post office.
Other customers were not aware he was holding a long-bladed knife at his side, which is clearly visible in the footage.
Police hope although the man tried to conceal his identity, someone can name him or may have seen him hanging around beforehand or running away from the post office, in High Street.
Det Sgt Becky Young of Braintree CID said: "We have carried out extensive enquiries but we have not been able to establish the identify of this man.
"He was clearly quite calm enough to queue behind other customers but luckily he appears to have lost his nerve when he tried to threaten staff.
"There is no doubt that he is a dangerous person who is prepared to carry a long-bladed kitchen knife for use in a robbery and we want to catch him before he can attempt this sort of crime again.”
The incident was at about 1pm on Wednesday, May 21.
Call Braintree CID on 101 or call Crimestoppers anonymously on 0800 555111.