She walked on, along the river path and up to the bench where the stroller was taken. GPS showed that Natasha had been walking along on the other side. What then? At some point, Jimmy Durrell had ridden past on his bike; Lauren had sat down on the bench, and fallen asleep; Natasha had gone back in the direction of her parked car. If only she’d not done that, and kept watching instead; she would have seen whoever did it.

Harper tried to put herself in the mind of the perpetrator. He or she was waiting nearby, somewhere within sight of the bench. Then, when Lauren fell asleep, they’d seized their opportunity, snatched the stroller, and made off. The path was narrow here, the roots snaking thickly across it. It wouldn’t have been easy to get the stroller away—the person must have been strong, determined, both.

When she reached the place where Natasha had been found with the babies she saw that it would just be possible to push a stroller across without it getting too wet, if you lifted it over the rocks. Natasha wasn’t strong enough to do that, from what Harper had seen; she’d got it stuck. But then, in the time between the taking and the returning, the sudden downpour had swelled the river, made it dangerous to cross.

On both banks, Harper could still see faint tyre tracks and footprints at the water’s edge. She pulled out her phone and studied the crime-scene photographs from the day of the abduction. In the images, on the far bank there were more than one set of tracks: one going in, one coming out. But, on the near side she could only identify tracks coming out, and those were probably made when Harper herself had hauled the stroller out of the water. In the photos from back at the bench, there were tyre tracks only on the bench bank, as if a stroller had been pushed in there, too. So, they knew that Natasha had pushed the stroller in from the opposite side, and Harper had pulled it out on the path side. The other tracks went in at the bench, and out at the river bend on the opposite bank. Harper considered it, and almost laughed. Swimming upstream with a stroller? Impossible. Perhaps it was impossible to know the truth. The rain and the swelling river had no doubt washed some tracks away before the photos were taken.