Erika parked outside the station, grabbed her bag and the answering machine and hurried down to the incident room. Moss, Peterson, Kay and the rest of the team were gathered around McGorry’s desk.

‘What is it?’ asked Erika, seeing the excited faces looking at her.

‘I’ve been working on all the statements with regard to Marissa Lewis’s death, and I’ve been putting together a timeline of the events on Christmas Eve,’ said McGorry. ‘She was working at the Matrix Club until eight-thirty. They had an early Christmas Eve show. No one hung around afterwards for a drink, and they all headed off for home. She took the 9.10 p.m. train from Charing Cross…’ He maximised a window on his computer monitor. ‘Here she is, running for the train, and just making it before the doors close.’ He played the short clip of Marissa Lewis running along in towering heels, her long coat flowing behind her. ‘She was alone when she got on the train.’

‘Okay,’ said Erika. ‘Does all this have a point?’

‘Oh yes,’ said McGorry with a grin. Peterson grinned and nodded too.

‘Well, get on with it!’

‘I also got footage from the train, when she changed at London Bridge. It’s a newer carriage and equipped with CCTV.’ They saw a crowded train carriage from the viewpoint of a camera mounted in the ceiling above the doors, looking down the carriage. ‘There she is, crushed in beside these two guys. Gay, I’m guessing, as they don’t seem to be paying any attention to her.’

‘Okay, okay, less of the personal comments.’

‘I’m just saying that there are no creeps who seem to be interested in her,’ he clarified, as he ran through the footage on the screen, showing the ten-minute train journey. ‘Okay, here we are at 9.42 p.m., and the carriage empties out at Forest Hill.’

‘Is there any footage from TFL of the station?’ asked Erika.

‘No. Nothing apart from the platform, and Marissa getting off with the rest of the crowds,’ he said, moving to another short clip.

‘Okay, what else do you have for me?’

‘This is the best. The school opposite Marissa Lewis’s house on Coniston Road has CCTV on two sides of the playground. One of them shows a view of Marissa Lewis’s front gate.’

The last video showed half of Marissa’s house, from the gate past the alleyway, and a portion of the street leading up to the junction.

‘What’s the time stamp on this?’ asked Erika.

‘This video is from 9.40 p.m.’

He scrolled through the black and white video, showing the empty snow-covered street, and the gate.

‘What’s that?’ asked Erika, when there was a flash of black at 8.51 p.m.

‘A cat jumping up on the gate,’ said McGorry.

‘Marissa had a cat,’ said Kay. ‘Beaker, its name is.’

‘Did you interview it?’ asked one of the uniformed officers.

‘Piss off,’ said Kay.

‘Quiet!’ said Erika.

‘Here we go,’ said McGorry. A figure in black, wearing a gas mask, walked into shot by the gate, moving carefully and purposefully along in the snow, almost staggering against the slippery surface. It reached the gate and looked up at the house. Then it carried on walking past the house, and stepped into the shadows of the alleyway.

‘Jesus,’ said Erika.

‘Okay, we run this forward for seven minutes,’ said McGorry, as the time stamp on the video whirred past. ‘There, you can just see Marissa Lewis arriving home.’

Marissa appeared at the gate. The room fell silent. Most of them had already seen the video, but the impact of it was just as striking the second time. Marissa opened the gate and went through, vanishing in the shadows of the front garden. Ten seconds later, the figure in the gas mask moved out of the shadows and approached the gate, carrying a long knife. It moved quickly through the front gate and was swallowed up by the darkness.

‘The camera doesn’t pick up anything that happened in the front garden,’ said McGorry. ‘Four minutes later, he comes back out.’

‘Are you sure there’s nothing?’ asked Erika.

‘I’ve watched it several times, slowed down. There’s nothing; the camera doesn’t pick up anything.’

He moved the video forward, as the figure came out, carrying the dripping knife. It stopped in the gate and looked back into the shadows.

‘He wipes it with a cloth, conveniently taken from his pocket. He stashes the knife in the pocket with the cloth, and then immediately turns to his right, leaving the shot.’ The team around Erika was silent. ‘I’ve lost him after that; there’s no CCTV in the residential area. He could have got in a car out of shot, or gone into a house; we don’t know.’

‘Run it back again,’ said Erika. She paused the video where the man in the gas mask emerged from the gate, and for a moment there was a clear view of the mask. She got up and went over to her desk, where she had a copy of the note sent to Joseph Pitkin. She held it up against the screen, looking at the hand-drawn gas mask in black biro ink.

‘Does this look like a similar kind of gas mask?’ she said.

‘I don’t know, it’s perhaps an old military gas mask,’ said McGorry.

‘We need to go back over the e-fit images given by the people who were attacked. And if there aren’t any, we need to go back and get them to work with an e-fit artist. Also, now we have this CCTV with the date and time stamp we could concentrate on a new door-to-door in the houses overlooking Marissa’s, in case anyone saw anything. This is great work.’

‘I was working with Kay,’ he said, grinning at her. One of the phones started ringing in the background and Moss hurried over to answer it.

‘This is good work, both of you.’

‘Boss,’ said Moss, holding her hand over her phone. ‘There’s been another attack by the man in the gas mask, in West Norwood, early this morning. A young lad on his way to work.’