A man walked, alone, into view, from the right side of the screen; tall, his hands deep in his pockets, swathed in layers, a hood over his head. His face looked strange in the black-and-white footage; it tricked the eye; Strike thought that he was looking at a stark white lower face and a dark blindfold, before reason told him that he was in fact looking at a dark upper face, and a white scarf tied over the nose, mouth and chin. There was some kind of mark, perhaps a blurry logo, on his jacket; otherwise his clothing was unidentifiable.

As the walker approached the camera, he bowed his head and appeared to consult something he drew out of his pocket. Seconds later, he turned up Bellamy Road and disappeared out of range of the camera. The digital clock in the lower right-hand portion of the screen registered 01:39.

The film jumped. Here again was the blurred view of the same intersection, apparently deserted, the same heavy flakes of snow obscuring the view, but now the clock in the lower corner read 02:12.

The two runners burst into view. The one in front was recognisable as the man who had walked out of range with his white scarf over his mouth; long-legged and powerful, he ran, his arms pumping, straight back down Alderbrook Road. The second man was smaller, slighter, hooded and hatted; Strike noticed the dark fists, clenched as he pelted along behind the first, losing ground to the taller man all the way. Under a street lamp, a design on the back of his sweatshirt was briefly illuminated; halfway along Alderbrook Road he veered suddenly left and up a side street.

Strike replayed the few seconds’ footage again, and then again. He saw no sign of communication between the two runners; no sign that they had called to each other, or even looked for each other, as they sprinted away from the camera. It seemed to have been every man for himself.

He replayed the footage for a fourth time, and froze it, after several attempts, at the second when the design on the back of the slower man’s sweatshirt had been illuminated. Squinting at the screen, he edged closer to the blurry picture. After a minute’s prolonged staring, he was almost sure that the first word ended in ‘ck’, but the second, which he thought began with a ‘J’, was indecipherable.

He pressed ‘play’ and let the film run on, trying to make out which street the second man had taken. Three times Strike watched him split away from his companion, and although its name was unreadable onscreen, he knew, from what Wardle had said, that it must be Halliwell Street.