The next day the local Leeds evening newspaper gave more information: ‘Mr Hobson said Chief Supt. Brook from Lancashire who is leading investigations into the murder of a prostitute – Joan Harrison – who was found battered to death in Preston in November 1975, was travelling from Blackpool to Leeds today to liaise with Leeds detectives.’ Wilf Brooks, the head of Lancashire CID, had his meeting with Hobson, then left a file on the Harrison case for the Leeds CID chief to examine, ‘because of the striking similarities’, according to the Evening Post on the 10th.
The similarities were indeed striking: twenty-six-year-old Joan Harrison had no convictions for prostitution but had been cohabiting with several men. She had once been a houseproud mother, devoted to her children, but then her life appears to have descended into chaos. She was prepared to have sex to fund her chronic alcoholism and addiction to cough mixtures containing small doses of morphine, often getting through as many as eight bottles a day. She suffered severe head injuries when she was killed in a garage in Berwick Road, Preston, less than a month after Wilma McCann was murdered. Her body was found lying face down, covered by her coat. Her clothing appeared to have been displaced in a similar way to the West Yorkshire killings. She had been wearing two bras, both unfastened. The inner one had been pulled up over her breasts, leaving the outer one in position. The left leg was outside her pants and tights, with the trousers replaced on both legs and partly pulled up. Finally, and most intriguingly, her left boot had been placed tightly between her legs with the zip opened. But there were also several differences. Robbery was thought to be a motive. Her handbag and some of her possessions were missing, and several injuries were thought to be due to kicking or stamping to the head and body. There were no stab wounds and it was thought she had had sex immediately before she was killed. Brooks himself was not convinced of a link between the killings, neither was Hobson. The Lancashire CID chief was more intent on finding the man who had normal and anal intercourse with Joan Harrison. Semen traces were identified as being from a blood group B secretor and police began a mass screening of local men in the Preston area.
In Leeds, Jim Hobson believed he had a more positive line to follow, the most important clue found in the three Leeds murders so far, and something the police could get to grips with: the tyre tracks left behind in muddy ground at the scene, almost certainly by the killer’s car. Scenes of crime officers quickly took plaster casts and calculations were made by the Harrogate laboratory. This herculean task of finding the car that left the tyre tracks began as a matter of urgency the very day Irene Richardson’s body was found. It became known as ‘the tracking inquiry’, because the initial goal was to ascertain the make of vehicle involved by measuring the track width of the vehicle from the distance between the tyre marks left at the scene. Only a certain number of cars would match that particular measurement. If the experts could narrow it down there was a good chance they might isolate the make of the killer’s car, and hopefully find it and, more crucially, the murderer.
As principal scientific officer at Harrogate, Ron Outtridge quickly contacted the murder incident room at Millgarth Street, the brand-new purpose-built police headquarters in Leeds which had been open less than a year. He had narrowed down the track width after a careful analysis of the plaster casts and photographs taken at the crime scene. The inner width was forty-six inches, the outer width fifty-four and a quarter inches. But there was one complication. It was impossible to tell whether the tracks had been made by the front axle or the rear axle. This doubled the number of vehicles involved. Outtridge had isolated the makes of tyres from the 250 different types of tyre available to motorists, but couldn’t determine whether the car had been driven straight on to the grass beside the pavilion or reversed in. All four were cross-ply tyres – one a Pneumant brand, manufactured in East Germany, worn down to 2 mm of tread, with an Esso E110 on the other side of the same axle; and there were two India Autoways tyres, both well worn, on the other axle. Each tyre had certain characteristics peculiar to its make, whether it was winter type, normal road use, remould, cross-ply or radial. In terms of mathematical probabilities, the chances of reproducing this exact combination of tyres were nearly 159 million to one. Another set of tyre tracks at the scene was eliminated. They were found to belong to a Leeds Corporation parks department trailer used to deliver chemicals to the pavilion three days before the murder.
By 5.15 p.m. on the day following the discovery of the body, Outtridge, with the help of the Home Office Central Research Establishment, had produced a list of a hundred different makes of vehicle which could fit the various combinations of these measurements. They were telexed to the Millgarth Incident Room as a matter of urgency. The twentieth vehicle on the schedule of makes was a Ford Corsair, which had a front track width of four feet two and a half inches. (Peter Sutcliffe was at the time driving a white Ford Corsair, which he had purchased second-hand shortly before he attacked Marcella Claxton.)
Jim Hobson wanted to move the inquiry forward quickly and instructed night duty officers throughout the Chapeltown Division to examine all parked vehicles during normal routine patrols. Every time a car was examined the officer took the registration number and tyre details, and a card was filled in showing that car had been eliminated from the inquiry. Other checks were carried out in scrapyards, vehicle breakers and auctioneers, and also of vehicles regarded as abandoned and due to be written off and crushed under the Civil Amenities Act. Tens of thousands of cars were checked in this way, but after six weeks the exercise was abandoned.
Hobson’s team had consulted an expert in the tyre industry, R. J. Grogan of Dunlop Ltd, who confirmed that the vehicle concerned was probably one of twenty-one makes and fifty-one models. His more rigorous and specialized analysis allowed the original list of a hundred models to be cut in half. The tyres fitted to one axle could have been of a diameter of twelve inches, thirteen inches or fourteen inches, while the other pair were of a type only manufactured in thirteen and fifteen inches. The science of determining which kind of vehicle was involved in a crime by analysing the combination of tyre tracks left at a scene was not perfect. A series of complex mathematical probabilities was involved. It was sometimes possible to determine the wheel base when a vehicle had sunk into soft ground when parked, or been reversed and driven off again. It might also be possible to measure the turning circle if the vehicle had been turned on its tightest lock, and again this could narrow down the car involved. However, it was unsafe to rely totally on measurements from front wheel tracks alone, as opposed to the rear wheel tracks, because of wear in the steering joints. A quarter of a century on, the situation is even more refined and tyre tracks can be as good as a fingerprint, according to one eminent forensic scientist: ‘The marks left by tyres when they are examined contain far more evidential value than would, say, a wound relating to the weapon that produced it. If you get a car with [a particular] track width and the turning circle and if that particular tyre is a Firestone, and there is a cut in it there – it’s virtually as good as a fingerprint.’
In a bold move Hobson got approval to carry out a mass screening of all vehicles of these models within the West Yorkshire police area and in the Harrogate division of the North Yorkshire police. It was a massive task and particularly arduous for the inquiry team involved, whose morale and motivation became seriously affected as time went on. Manual searches were carried out at vehicle licensing offices and a search was made of computer records by the Police National Computer (PNC) unit. Hobson learned that 53,000 vehicles would need to be examined. Then there was another problem. The Police National Computer was a relatively new resource, first introduced in 1974. Today it employs twenty-first century technology and there is a second-generation PNC which deals with 200,000 queries a day from British police forces. A vast array of information has been loaded into the system, which includes 42.5 million vehicles registered in the United Kingdom and their owners; stolen property; criminal records; missing and wanted people and disqualified drivers; and people subject to court orders.
But in 1977 the PNC was still in its relative infancy, and programmers were continuing to input the data to build the vast index of vehicles and their owners. A great deal of ‘back record conversion’ had yet to be done, transferring the records of the Driver and Vehicle Licensing Centre in Swansea on to the Police National Computer. Some information was held in the PNC, but to be sure they would not miss anything Hobson’s team also had to check manually with the local Leeds Vehicle Licensing Office. Sutcliffe’s white-coloured Ford Corsair – KWT 721D – was among the