Four incidents across West Lothian and Fife where a man threatened post office staff with a knife were reported to officers on Monday.
In the first case, a man stole a sum of cash from the till of a post office in Linlithgow at about 11.25am.
At about 12.15pm, there was an incident reported in Cowdenbeath High Street where a man tried to get behind the counter of a post office but was refused and left the premises.
Then, around half an hour later, a man entered a post office in High Street, Burntisland, with a knife and made off with sum of cash.
In the final incident, in Elphinstone Street, Kincardine, a man attacked a female shop worker before making off with money.
The suspect in each incident is described as white, in his 30s with a stocky build, stubble and wearing dark clothing and a high-visibility jacket.
Detective Inspector Colin Robson said: “These incidents were all extremely alarming for the staff involved and we are providing each of those affected with all the support they require.
“We believe the same male was responsible for each of these crimes and would ask that anyone who can help us to identify this individual contacts police immediately.
“Anyone with any other information relevant to this investigation is also urged to get in touch.
“We are also working closely with Post Office counters with a view to raising awareness and prevent further crimes.”