Petra stood up and paced the floor. ‘Let’s look at what we’ve got. We have the car that Dr Schilling’s boyfriend saw and a matching car with Hamburg plates near the scene of de Groot’s murder, which gives us Wilhelm Mann. We have a smear of marine engine oil on the folder he left in Pieter de Groot’s filing system …’

‘And no forensics from any of the other three recovered files,’ Marijke chipped in gloomily.

Petra continued undaunted. ‘We also have a sailor’s knot, which leads back to Wilhelm Mann.’

‘And thousands of other people,’ Tony pointed out.

‘Thank you, Tony,’ Petra countered with a sardonic smile. ‘Thanks to the work the river police have been doing over the last week, we can put the Wilhelmina Rosen at or near all four murders, which also gives us Wilhelm Mann. We have a killer who uses the alias Hochenstein. Tony’s list from Schloss Hochenstein gives us an Albert Mann who was a child survivor of psychological experiments.’

Marijke butted in. ‘Yesterday we heard from the cops in Hamburg. They did a records search on Wilhelm Mann which gives him a grandfather called Albert Mann with the same date of birth as the man on Tony’s list from Schloss Hochenstein. He died two years ago. The inquest said it was an accident, but if you look at it with the idea that his grandson is a killer, it is not hard to see that it could have been murder.’

‘Christ, with that much circumstantial evidence, why don’t Köln just bring him in for questioning? I would,’ Petra complained.

‘It wouldn’t do any good,’ Tony said. ‘I doubt he’d say anything.’

‘So what do we do?’ Marijke said plaintively.

There was a long silence. Petra threw herself down on the sofa, making Tony flinch. He gritted his teeth and said, ‘I think I could break him.’

‘They wouldn’t let you interrogate him,’ Petra pointed out.

‘I’m not talking about a formal interrogation,’ Tony said. ‘I’m talking about me and him, one to one.’

Petra shook her head. ‘No way. You’re not fit enough for anything like that. He could kill you like snapping a stick.’

‘I’m not that pathetic,’ Tony said. ‘I’ve been moving around a lot more today. The painkillers are starting to kick in. I can do it.’

‘I thought you said his English was poor,’ Petra objected.

‘Ich kann Deutsch sprechen,’ Tony said.

Petra stared at him open-mouthed. ‘You kept very quiet about that.’

‘How do you think I managed to read the case files?’ He dipped his head at Marijke in acknowledgement. ‘I was very grateful that you had your material translated into German, because I really can’t manage Dutch.’

‘It’s still far too risky,’ Marijke said.