Wesley got up and went to his suitcase. He opened it and pulled out a thick roll of white paper. Removing the rubber band, he spread open very long drawings of New York’s subway system that included all tracks and structures, every manhole, trash can, car marker, platform edge. The diagrams covered most of his office floor, some more than six feet long. I studied them, fascinated.

‘This is from Commander Penn,’ I said.

‘Right,’ Wesley replied. ‘And what’s on her computer is even more detailed. For example’ - he squatted, pointing and moving his tie out of the way - ‘in March of 1979, turnstiles at CB number 300 were removed. That’s right here.’ He showed me on a drawing of the 110th Street station at Lenox Avenue and 112th Street.

‘And a change like that now,’ he went on, ‘goes directly into the Transit Police computer system.’

‘Meaning that any changes are instantly reflected on the computerized maps,’ I said.

‘Right.’ He pulled another drawing closer, this one of the Eighty-first Street Museum of Natural History station. ‘Now the reason we think Gault is using these maps is right here.’ He tapped an area on the field survey that indicated an emergency exit very near Cherry Hill.

‘If Gault was looking at this drawing,’ Wesley went on, ‘he most likely would choose this emergency exit as the one to come in and out of when he committed the murder in Central Park. That way he and his victim could travel unseen through the tunnels after leaving the museum, and when they surfaced in the park they would be very close to the fountain where he planned to display the body.

‘But what you don’t know from looking at this three-month- old printout is the day before the murder, the Maintenance of Way Department bolted that escape door shut for repairs. We think that might be why Gault and his victim started out closer to the Ramble,’ he said. ‘Some footwear impressions recovered in that area, as it turns out, are consistent with theirs. And the tracks were found near an emergency exit.’

‘So you have to ask how he knew that exit in Cherry Hill was bolted shut,’ Marino said.

‘I suppose he could have checked it first,’ I said.

‘You can’t do that above ground because the doors don’t open except from inside the tunnels,’ Marino said.

‘Maybe he was down in the tunnel and saw from the inside that the door was bolted,’ I argued, because I sensed where this was leading and did not like it.

‘Of course that’s possible,’ Wesley reasonably said. ‘But Transit cops go down into the tunnels a lot. They’re all over the platforms and the stations, and none of them remembers seeing Gault. I believe he travels down there by computer until it suits his purposes to make an appearance,’

‘What is Lucy’s role?’ I asked.

To manipulate,’ Marino said.