Detectives investigating Jennifer Cardy's murder trawled 560,000 old fuel receipts in a bid to nail Robert Black - eventually they found the one that proved crucial.
On the face of it a credit card docket for a white Datsun van taking on petrol in Coventry would not appear relevant to the disappearance of a schoolgirl in Northern Ireland.
But it was absolutely vital in a cold case with little evidence to place the serial killer, who then worked as a dispatch driver based in London, near the scene of the crime.
It proved that Black, who scrawled his name on the receipt, was driving south to London the day after the murder in August 1981 in a van primarily used to deliver posters in Northern Ireland.
That, argued the Crown, could only mean one thing - he was on the way back to base after disembarking from the overnight ferry from Belfast at Liverpool docks.
The receipt was found among reams and reams of microfiche retrieved from storage warehouses at the UK headquarters of Shell UK in greater Manchester.
Similar dockets were hugely significant in Black's triple murder trial in 1994.
The Coventry slip was one small piece of paper among 22 tons of documentation transported to police in Northern Ireland when the case was reopened in 2002 by current detective superintendent Raymond Murray.
Two articulated lorries were required to transport the files to the region.
Hundreds of boxes, relating to all Black's past crimes, now fill one and half secure storage rooms in a Belfast police station.
They were initially based in Lisburn, where Mr Murray started re-examining the documents with help from officers from a Major Investigation Team (MIT).
Experienced detective chief inspector Stephen Clarke played a key role.
When Mr Murray was redeployed to Londonderry, Mr Clarke took on the day to day running of the investigation from a new base in Belfast alongside detective constable Andy Macleod.
Aided by a small team of other officers, they painstakingly analysed all the evidence in an effort to build the case against Black.
Detective Constable Yvonne Younger also had a vital role, particularly when it came to keeping the Cardy family aware of developments.
Throughout the trial in Armagh Crown Court she was ever present at the side of Jennifer's parents Andy and Patricia in her role as family liaison.
Mr Clarke retired in 2007 after 29 years of service but was re-employed as a specialist consultant as the case headed to trial.
In totality, the Black files contained references to 187,000 individuals, including 60,000 witness statements.
In all the murder investigations to date around 2,500 persons of interest were eliminated as suspects.
It took nine years to construct the case that finally ended with Black being found guilty of Jennifer's kidnap and murder.
Little wonder when the senior detectives were wondering what to christen the investigation, one name seemed obvious: Perseverance.