An automatic door at the front of the building society was kicked in

Police believe Joseph Nwabuko, 28, may have been trying to note down the registration number of the getaway car.

Mr Nwabuko from Birmingham, West Midlands, was shot by robbers fleeing a Nationwide Building Society in the city centre at 2320 GMT on Monday.

The men snatched an unknown quantity of cash from Securicor guards and escaped in a car.

�150,000 reward

Securicor has offered a �100,000 reward for information leading to the conviction of the robbers and the Nationwide another �50,000.

Mr Nwabuko, who moved to Birmingham from Walsall about a month ago, died in an alley close to the city's New Street Station and the Bullring shopping complex.

Detective Chief Inspector Nigel Rock of West Midlands Police described how the victim followed the men after they robbed guards restocking cash machines.

He said: "He may have been going out of curiosity or maybe he even recognised the gang members.

"Clearly it is a brutal and totally unnecessary offence. It appears as if the offender with the gun had plenty of opportunity to get in the car and make his escape.

"For whatever reason he has chosen not to and returned to Joseph and quite deliberately shot him."

It's absolutely deplorable, it's absolutely despicable Steve Hayes, Nationwide branch manager

Mr Rock confirmed the killer may have been as far as 40 yards away from the scene of the shooting when he decided to return from the car.

Asked to elaborate on the exact circumstances of the killing, Det Chief Insp Rock added: "It would appear as if Joseph has been in the area and has followed them down that ramp.

"The indication is that Joseph walked at a normal pace down the ramp. What we don't know is why Joseph has chosen to follow these robbers."

A post-mortem examination showed Mr Nwabuko died from a gunshot wound to the chest, after a round passed through his left arm and into his lungs.

'Unusual' offence

Earlier Chief Inspector Steve Glover said two men had managed to kick the building society's automatic door in. He described the offence as "unusual".

He said: "The time of day is unusual, the location is unusual and they way they've gone about it is unusual."

Mr Nwabuko's family are understood to still live in the Walsall area.

Branch manager Steve Hayes said: "It's absolutely deplorable, it's absolutely despicable."