Essex Police are trying to identify a man who queued up at a post office and then threatened staff with a large kitchen knife.
The hooded man waited behind two customers at the post office in Thaxted near Stansted, before pulling out the long-bladed knife. He tried to rob staff but ran off empty-handed.
It happened at 1pm on Wednesday 21st of May.
The man was caught on CCTV and police are hopeful someone may recognise him or may have seen him near the post office last week.
Detective Sergeant Becky Young said: "We have carried out extensive enquiries but we have not been able to establish the identify of this man.
"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."
Anyone with information is asked to contact police on 101 or call Crimestoppers anonymously on 0800 555111.