Of course the abduction of Mandy Smith made Black a prime suspect for Hector Clark, as the MO was strikingly similar to that in the cases of Susan, Caroline and Sarah.    When Clark first saw Black following his arrest in July 1990 he remembers,

“Slowly he looked up at me and my gut feeling was that this was my man.  I had always thought that when I saw him I would know him and every instinct told me this was the guy.  I knew by his body smell and his dishevelled appearance.  Except that he was bald, he was just as I expected.”

But "gut feeling" and "instinct" are not good enough.  In spending so much time analysing such crimes, the police inevitably start to feel that they know the offenders in certain ways.  They think they know what they will look like and how they will behave.  George Oldfield, heading the Yorkshire Ripper inquiry, similarly said on several occasions that if he were in a room full of potential suspects he would instantly 'know' his man.  But as the Ripper investigation showed us, this is a dangerous assumption.  Peter Sutcliffe was interviewed nine times during the course of the five-year investigation, but nobody 'recognised' him.

In the hope of eliciting some incriminating evidence, the police decided to interview Black.  As he was already serving a life sentence they thought that he might be willing to talk about any other crimes he had committed.  Interviewed in Scotland, Black talked candidly to officers about the offences for which he had previously been convicted, for the best part of six hours.  He was frank about a variety of topics, including his one proper relationship with a woman, his attraction to little girls, the sexual abuse he had endured as a child, his fantasy life, and his masturbatory practices.  Eventually however, when the officers asked Black about his work with Poster Dispatch and Storage and his whereabouts on the day of Caroline Hogg’s abduction, he fell silent.  When it came to the abductions and murders of the three little girls, Black would simply not talk to the police.

It was apparent that the police would have to find their evidence the hard way, through old-fashioned, painstaking detective work: they were going to have to look at Black's life over the past eight years.  In most cases the tracing of a person's daily movements over the past decade would prove an impossible task, but in this case the police were fortuitous due to the nature of Black's work.  From a careful examination of work records, wage books, and receipts from fuel credit cards, the police were able to begin tracing Black's life.

Susan Maxwell's abduction had taken place in Coldstream on 30 July 1982.  It was the task of the police to establish where Black was at every stage during that day.  The first step in the process was to see whether PDS had records of journeys carried out by drivers dating that far back.  The police were initially dismayed to find that potentially vital company records had been destroyed just months beforehand, as was company policy after a certain length of time had elapsed.  Yet new hope arose when it was established that the wage books from that time were still available.  As different runs command different wages it was established - from the amount of money that Black received in his pay - that he must have done the London-Scotland run sometime between 29 July and 4 August.

The time still needed narrowing down, however.  The police next looked at petrol receipts from the company's fuel credit cards that all drivers carried and it was established that Black had been in the Borders area on 30 July.  He had filled up his white Fiat van just south of Coldstream before the time that Susan was snatched, and just north of Coldstream after the time of her abduction.  The quickest route between the two garages was the A687, directly through Coldstream.  Black had previously told his work-mates that when returning from a Scottish-run he preferred not to take the most direct route (which was the M6 to the M1) but to get to the M1 via the A50 through the Midlands.  Susan's body was found by the A518 in Staffordshire, not far from the junction for the A50.

The case against Black for the murder of Caroline Hogg was built in a similarly meticulous fashion.  On 8 July 1982, the day of Caroline's abduction, it was established that Black had delivered posters to Mills and Allen in Piershill, just over a mile north of Portobello.  Petrol receipts showed that he had filled up at a petrol station in Belford, Northumberland, on this day and that the most obvious route from Belford to his delivery point in Piershill was through Portobello.  The post-mortem had found that Caroline's body had been kept by her killer for four days after her abduction - dead or alive, they could not determine - making the 12th the first day on which her body could have been disposed.  On this day Black had delivered posters to Bedworth, just over ten miles from where Caroline's body was found. 

The circumstantial evidence for the case of Sarah Harper was equally strong.  On 26 March, the day of her abduction, Black had delivered posters to a depot just 150 yards from the place that Sarah was last seen.  Petrol receipts from the next day put Black as driving directly past the spot on the A453 to Nottingham where Sarah's body had been deposited.

In addition to the growing mountain of circumstantial evidence another incident came to Clark's notice.  On 28 April 1988, 15-year-old Teresa Thornhill had been to the park with some friends.  Teresa walked part of the way home with one of these friends, Andrew Beeson.  Just after she and Andrew had gone their separate ways, Teresa noticed that a blue van had stopped just ahead of her on the opposite side of the road; the driver had got out and was looking under the bonnet.  As she approached, the man shouted to her: "Can you mend engines?"  Uneasily she replied that she could not and walked on.  The next thing she knew, the man had grabbed her from behind, picked her up and was carrying her across to his van.  She said later:

"I will never forget his hairy arms, sweaty hands and smelly T-shirt.  He came over to me and got me in an all-encompassing bear hug which I could not get out of because he was very strong.  I tried to struggle free and began shouting for my mum.  I was looking around for something to hit him with, but there was nothing there.  Then I grabbed him between the legs."

She also knocked his glasses to the ground, screaming all the while.  Teresa's friend Andrew heard her screams and ran towards the van shouting, "Get off her, you fat fucking bastard."  Teresa's struggle and Andrew's timely arrival meant that her attacker had little choice but to drop his victim and make his get-away.

Unfortunately, at the time there was nothing to obviously link Teresa's attack to the abductions and murders of Susan, Caroline and Sarah.  Most importantly, these girls were aged between five and 11, whereas Teresa was 15, nearly a woman.  Teresa looked far younger than her years, however: she was under five feet tall, with a girlish figure, and wore no make-up.  She did not look like a teenager.  If this had been taken into account at the time, the abductions would have seemed remarkably similar.  If this case could be shown to be linked to the murders, then it was an important breakthrough as Teresa's description of her attacker and his van matched Black exactly.

By the end of 1990 the police had gathered a mass of circumstantial evidence against Black, but unfortunately they had no forensic evidence and no confession.  They decided to re-interview Black more rigorously, but for three days he refused to answer any of their questions, as was his right.  The police had no real choice but to proceed with what they had.  In May 1991 the police submitted their report to the Crown Prosecution Service who would decide whether to go ahead with a prosecution.  In April 1992 Black was served with ten summonses.