Days after the violent raid which left a security guard fighting for his life, no one has been arrested.

Detectives were at the London Road supermarket yesterday morning (Tuesday, September 23) - exactly one week after the robbery - to question shoppers.

They hope it will spark people's memories of any suspicious activity that could help officers catch the dangerous gunmen.

At around 7.30am on Tuesday, September 16, a security guard from Group Four Securicor (G4S) was ambushed, as he filled the store's outside cashpoints, by a man wearing a balaclava and wielding a handgun.

The guard was shot in the leg and a shopper who intervened was also gunned down.

A team of more than 50 officers are working to piece together evidence, including trawling through CCTV footage.

Investigating officer Detective Chief Inspector Michael Hanlon said: "There is a large number of inquiries that we are currently pursuing. There is a wealth of CCTV that we need to sieve through and one of the key lines is the fact that a very large reward has been offered jointly by G4S and Tesco."

The firms have put up £25,000 for information leading to the arrest, charge and conviction of the raiders.

The two robbers fled on a stolen black Triumph motorbike, registration P354 JYB, with a 'substantial amount' of cash in a container. They dumped the bike in nearby Treehanger Close, Tring, before making off in a small white van.

Detectives at the store yesterday were asking shoppers if they had seen the distinctive bike, which has the word Tiger written on the side, or the getaway van at anytime before or after the attack.

DCI Hanlon said: "The purpose of us being there was not only to identify any witnesses from last week but quite clearly the offenders will have observed both the Tesco store and the G4S van in their planning.

"Anybody who saw anyone acting suspiciously during the weeks leading up to the attack I would urge to make contact with us."

He added that this was not a 'spur of the moment' attack.

The motorbike was reported stolen earlier this month (September) from a location in East London and officers are trying to trace the vehicle's movements after it was taken.

Tests are being carried out on bullets and cartridges recovered from the raid and Herts Police are working with neighbouring forces to see if there have been any other similar offences that could be linked.

The 46-year-old security guard from Harrow has undergone several operations at Stoke Mandeville Hospital in Aylesbury and
remains in a serious but stable condition.

Meanwhile, the shopper, aged 33, from Hemel Hempstead, who was also shot in the leg has returned home from hospital.

He works at the Arts Educational School in Tring and is believed to be a maintenance worker. Staff at the performance school have been in contact with him.

DCI Hanlon said: "Once the robbery had started the member of public came to the assistance of the guard.

"He did what he considered to be right in those circumstances. It was a split second decision and unfortunately it has led to him sustaining serious injuries."

The two cashpoints outside Tesco remain cordoned off by police tape and officers are still carrying out interviews with store staff.

Anyone with information about the robbery is asked to call police on 01707 354432 or call Crimestoppers anonymously on 0800 555 111.