Boiling water killers caught by 'glimpses' on CCTV
They denied being there on the night of the murder - but the work of a detective piecing together CCTV clips like a jigsaw puzzle proved they were lying.
Two men convicted last week of the murder of Sonny Grey said they were nowhere near Westwick Gardens in Lincoln when the 70-year-old was beaten, throttled and had boiling water poured over him.
With no forensic evidence to link the killers to the crime, it seemed an uphill task for Lincolnshire Police to prove otherwise.
But, in the end, the killers' movements were tracked by one policeman - painstakingly, piece by piece - from the fragments of CCTV footage.
Alone, these 25 clips - some containing just split-second glimpses of the men - were almost meaningless.
But together, they were enough to help the jury at Nottingham Crown Court to convict two killers.
The case showed how the vast reams of CCTV footage being collected every day by Britain's network of cameras can become vital evidence - even if they do not capture the crime taking place.
Race against time
The Sonny Grey case was Det Con Adam Battersby's first murder investigation.
Normally based at Grantham, Mr Battersby, 36, joined the team working on the case on 30 October 2011 - the day after Sonny Grey was attacked.
His first job was to compile as much CCTV from the night of the crime as possible, from sources as diverse as fast food outlets, petrol stations, traffic monitors and people's homes.
He and his colleagues recovered footage from hundreds of cameras - approximately 300 in Lincoln alone.
The detective soon realised he was in a race against time. Many outlets only keep their footage for a month - some less than that. One shop he visited on day four of the investigation had already wiped their cameras.
To make sure he had not missed any potential evidence, Mr Battersby walked around the scene of the crime, on the look-out for cameras.
The investigation soon focused on three men - Rocky Curtis, 25, and Robert Holmes, 24, of Wisbech, Cambridgeshire, and Curtis's brother Tom. Tom Curtis was arrested in connection with the murder, but later died.
Mr Battersby was asked whether it was possible to track Tom Curtis's Volkswagen Toureag on its journey from Wisbech on the night of the crime.
He went online and noted down all the petrol stations on the route the car had travelled, from Wisbech to Lincoln.
He then visited them each in turn, collecting hours of footage, all of which needed to be watched again and again for clues.
What followed was a gruelling 14 months of detective work.
'A lonely existence'
Mr Battersby spent nine hour days shut away in an office in Lincoln police station studying the footage.
"We used to call it my little broom cupboard," he said.
"I think people forgot I existed! It's quite a lonely experience at times."
Some of the cameras had better-quality footage than others. By using those good-quality cameras as reference points, Mr Battersby was able to map the journey the car had taken on the night of the murder.
He even drove the route himself, almost exactly a year after Mr Grey's death, to check the timings made sense.
By the end of the investigation, he was able to produce 25 separate clips showing the car setting out from Wisbech, driving towards Lincoln and parking round the corner from Sonny Grey's home.
The footage even showed the killers walking towards the scene of the crime, then running back to their car, before stopping for snacks at a petrol station on the way home.
The clips proved decisive in the case against the men.
As judge Mr Justice Saunders gave life sentences to Holmes and Curtis, he praised Mr Battersby's efforts, describing them as "not only painstaking but, in my view, remarkable. It eventually enabled the police to solve this crime."
"Some of the clips were just a second long," said Mr Battersby. "If I had blinked or yawned, I would have missed them."
CCTV is not without controversy and the detective agrees it needs to be used carefully. "At times, it can be intrusive," he said.
But it is, he says, becoming increasingly valuable as a means of catching criminals.
"CCTV used to be an afterthought," he says. "If you were conducting an investigation, you would think, 'Oh - I wonder if there is anything on camera?'
"But now it's hugely important. In this case, no matter how many times the defence said 'That's not them', the jury could look for themselves and make their own minds up.
"CCTV is a witness that can't get things wrong. Humans are susceptible to error. But, if something is on CCTV, it's there for all to see."