A reward of up to £25,000 has been offered for information following the robbery and attempted robbery of two Nottingham security vans.
It comes after a robbery outside the Co-op store in Trowell Road, Wollaton, on 21 October.
Six days later there was an attempt to raid a security guard delivering cash to a store in Beech Avenue.
Police have appealed for more information on the movements of a car used in both incidents.
Three men, two black and one white, used a green Volkswagen Passat, R954 ONL, in the armed raid on the Post Office guard in Beech Avenue.
It was later found in nearby Herbert Road and police said the gang had got into a second vehicle soon afterwards.
The security guard suffered a wound to the leg after one of the offenders discharged a firearm.
A Post Office Ltd spokesman said: "This was a shocking and horrific attack on a Post Office worker who was carrying out her duty.
"We are offering a substantial reward for information leading to the arrest and conviction of those responsible and would urge anyone with information to contact police."
A police spokesperson said: "We are still trying to piece together where the VW Passat was driven and stored during the time between the robbery and attempted robbery.
"Someone out there holds this information and the offer of a reward may encourage them to come forward.
"The suspects used extreme threats and violence during both offences, with no regard for the potential consequences.
"We need to find whoever did this and would urge the public to tell us if they know anything."