Finn carried the mugs through to the living room, and rested Kate’s on the coaster on the coffee table. ‘What is it? You look like you’ve just seen a ghost.’

Kate glanced up with a mad look in her eyes. ‘I figured out what was wrong with the re-enactment: make-up!’

‘Make-up? I don’t understand.’

She pointed at the screen. ‘Amy was found naked at the scene, right? So the director had to craftily shoot the crime scene for the re-enactment, so as not to show any nudity on the screen. But look at the actress’s face. What do you notice?’

He studied the screen, before sighing. ‘I give up. Apart from the fact she doesn’t look much like Amy.’

‘It’s the make-up. The actress is wearing eyeliner and dark lipstick, which is how we know Amy was dressed up on that night, from the footage we have of her in Waterloo station.’

‘And?’

She handed Finn the crime-scene photograph. ‘When we found her, she wasn’t wearing any make-up.’

‘So? Maybe she took it off.’

Kate clapped her hands together excitedly. ‘Exactly!’

Finn still wasn’t sure what she was getting at.

Kate rolled her eyes. ‘The scene of crime officers suggested there might have been a struggle inside the flat before Amy was killed, but they couldn’t say conclusively that the mess they found was as a result of the killer attacking Amy. You said yourself, she wasn’t the neatest of homemakers.’

He smiled at the memory. ‘Yeah, her place was usually a bit of a pigsty.’

‘But the fact that she removed her make-up before she was killed tells us that she was in her flat for an extended period before he struck.’

His eyes wrinkled with confusion. ‘How does that help us figure out who did it?’

She took a large gulp of coffee. ‘It means she was with the killer before he struck. I believe she invited him in, and she must have felt comfortable enough with him being there to remove her make-up.’

‘So, because she removed her lippy, you think that means she knew her killer? How do you know the killer didn’t wipe it off afterwards? He cleaned his DNA off the other victims with some kind of cleansing wipes, right? Maybe he did the same to Amy?’

‘Although traces of ammonia were found on Amy’s body, none was found on her face. It’s in the pathologist’s report. In fact, it even mentions the presence of a moisturising cream on her cheeks.’

Finn perched on the arm of the chair. ‘But if you’re right, then it means Brookes definitely wasn’t the killer.’

She nodded grimly. ‘Just stick with me for now. I don’t think she would have invited someone like Brookes up. You’ve seen his picture.’

‘Maybe that’s why she removed her make-up: to make him feel more comfortable with his own appearance?’ Finn was growing increasingly agitated. ‘Or maybe Brookes was wearing a wig and make-up himself, to look more normal.’

Kate tilted her head. ‘I know this is difficult for you to hear, but I think Armitage’s team are right. I think whoever killed Amy dressed the scene to mirror Brookes’s MO.’ She paused. ‘I need to go back to the beginning. If Amy knew her killer, then maybe she’d been on a date with him before. I need to find the original list of suspects.’

She reached down into the pile of papers at her feet.

Finn stood and picked up his coat and scarf.

She took another swig from the mug. ‘Where are you going?’

He looked solemnly at her. ‘I should get back. My dad will be wondering where the hell I am. You need to focus anyway, and I’d only be a distraction.’

‘Please, Finn, don’t be despondent. I know that separating this crime from the other three was a blow, but actually I think it puts us a step closer to finding out the truth.’ Kate was filled with renewed energy. ‘It might take us a bit of time, but we’ll find him. I promise you that. You don’t need to take off. In fact, you could really help me if you stick around. I have a ton of questions that need answering. For example, where did the killer get the murder weapon from? Did he bring the knife with him? If so, was he always planning to kill her? Also, how did he manage to find out so much about Brookes’s MO? How did he know to wipe her body down with wipes? That bit never made the press…’ Her words trailed off.

Finn blurred as thoughts raced through Kate’s mind.

Someone Amy felt comfortable getting a lift from – comfortable enough to invite them into her flat.

Someone who knew enough about the previous murders to dress the scene in the same way.

Someone who left no DNA in the flat, or whose DNA presence would be dismissed.

Finn’s face came into focus as her blood ran cold. ‘When we spoke in London on Saturday, you said Amy didn’t tell you anything about the investigation. How did you know that Brookes wiped down his victims with an ammonia-based product?’

He stared down at his feet as he reached into the inner pocket of his coat.

Kate’s heart skipped a beat as she saw the blade.