And yet this primitive mental filing system was as chaotic as it was effective, pictures hanging askew, books stacked any which way on the shelves with dirty cups and glasses squeezed into the gaps, the floor covered in a confetti trail of newspaper cuttings and half-read books left facedown, alongside a stack of index cards inscribed with notes for a forthcoming lecture. And while a favoured few of his artefacts had been placed in a glass display cabinet, the rest were scattered indiscriminately around the room, some squeezed on to his desk and the marble mantelpiece, others lining the edges of the bookshelves like paratroopers waiting for the order to jump.

Despite his cheerfulness on the intercom, Aurelio now seemed to have sunk into what Allegra could only describe as a sulk, his bottom lip jutting out, brows furrowed. Funny, she thought, how old age seemed to have given him an almost childlike ability to flit between moods on a whim.

‘Maybe you shouldn’t come any more,’ he sighed. ‘Spend time with your real friends, instead, people your own age.’

‘Don’t start that again,’ she sighed. ‘I’ve told you, I’m too busy to have any friends. Besides, I like old things.’ She winked. ‘They smell more interesting.’

Approaching seventy, Aurelio had no family left now, apart from a distant cousin who only seemed to show up when he needed a handout. As they had got to know each other, therefore, Allegra had taken it upon herself to look in on him whenever she knew she would be in the area. And sometimes, like today, when she knew she wouldn’t.

‘But you said you’d be here for lunch,’ he continued in a hurt tone, although she could sense that her reply had pleased him. ‘You’re late.’

‘And whose fault is that?’

He grinned, his sulk vanishing as quickly as she suspected it had appeared. He had a kindly face, with large light brown eyes, a beaked nose and leathered skin that spoke of too many long summers spent hunched over an excavation trench. He was dressed in an open-necked shirt and a yellow silk cravat, another hangover from his Oxford days. As ever, he was wearing a motheaten grey cardigan for warmth, his refusal to pay the ‘extortionate’ prices demanded by ‘piratical’ energy companies condemning his apartment to a Siberian permafrost for at least three months of the year.

‘So they did call you?’ he crowed.

‘I knew it!’ she remonstrated angrily. ‘Who did you speak to? What did you tell them?’

‘The GICO wanted an antiquities expert. They called the university. The university put them on to me. I told them I’d retired and recommended you instead.’

‘Did they tell you what they wanted?’

‘Of course not. It’s the GICO. They never tell you anything.’ He paused, suddenly concerned. ‘I thought you’d be pleased.’

With a deep breath, Allegra recounted the events of the past twenty-three hours. The inverted crucifixion at the site of Julius Caesar’s assassination. The carefully staged beheading in the Pantheon. The apparent link to two Caravaggio masterpieces. Aurelio listened intently, shaking his head at some of the more gruesome details, but otherwise remaining silent until she had finished.

‘So the man they found in the Pantheon…?’

‘Was Annibale Argento’s twin brother, Gio.’

‘Merda,’ he swore, for what could well have been the first time since she’d known him. ‘They must be lapping it up.’

They, she knew, referred to the media, an industry he despised, having been tricked a few years ago into authenticating a forged Etruscan vase by an investigative reporter. He gave a contemptuous wave of his hand towards an imagined TV set in the corner of his room, as if trying to further distance himself from an object he had already demonstrably banished from his life.

‘I’ve spent half the day trying to see if there’s anything else that links the two sites or any of Caravaggio’s other works. Gallo is trying to get me seconded on to the case full time.’

‘I’m sorry, Allegra. I didn’t know…I didn’t mean to get you involved in anything like this.’

She shrugged. It was hard to be angry with him. It was Aurelio after all who, guessing that she would quickly tire of academia, had encouraged her to apply to the art and antiquities unit of the Carabinieri in the first place. He’d only been trying to help.

‘I know.’

‘Anything to go on?’ he asked hopefully.

‘Plenty to go on. Just no idea where to start,’ she sighed. ‘Which reminds me. There’s something I wanted to ask you.’

‘Anything, of course.’

‘Both victims had what looked like an antique coin in their mouths.’

‘To pay Charon,’ Aurelio guessed immediately.

‘That’s what I thought. Except it wasn’t a coin. It was a lead disc.’

‘Lead?’ Aurelio frowned. ‘That’s unusual.’

‘That’s what I thought. I seem to remember reading that Roman forgers used to fake coins by casting them in lead and covering them in gold leaf, but I wondered if there was some other reference to the Classical world that might.’

‘Unusual, but not unprecedented,’ he continued, interrupting her. ‘Can you reach that red book down for me.’

She extricated the book from between the fifteen or so other academic texts he had written and handed it to him. He held it for a few seconds, his eyes closed, fingers resting lightly on the leather cover as if he was reading braille. Then, opening his eyes, he leafed through it, the brain haemorrhage that he’d suffered some fifteen years before betraying itself in his slow and deliberate movements.

‘Here,’ he fixed her with a knowing smile, about halfway in.

‘Here what?’

‘Threatened by the Persian empire, several Greek states came together in the fifth century BC to form a military alliance under the leadership of the Athenians,’ he read. ‘Members had to contribute ships or money, and in return the alliance agreed to protect their territory. Symbolically,’ he paused, Allegra remembering that he used to employ the same theatrical technique in lectures when he was about to make a particularly compelling point. ‘Symbolically, upon joining, representatives of the member states had to throw a piece of metal into the sea.’

‘Lead,’ Allegra breathed. He nodded.

‘Normally a piece of lead. The alliance was to last until it floated to the surface again.’

There was a pause, as she reflected on this.

‘And you think…?’

‘You asked about a link between lead and the Classical world.’ He smiled. ‘Thinking’s your job.’

‘What was the name of this alliance?’

Aurelio pretended to consult the book, although she could tell it was just an excuse for another of his dramatic pauses.

‘They called themselves the Delian League.’

J. Edgar Hoover Building, FBI headquarters, Washington DC

18th March – 9.37 a.m.

The door buzzed open. Tom didn’t bother to look round. He could tell from Ortiz’s shuffling steps and Stokes’s heavier, wider stride, who it was.

‘How long are you going to keep me here?’ he demanded angrily.

‘A federal agent’s been killed, Mr Kirk,’ Stokes replied icily, no longer even attempting to mask his instinctive hostility. He dragged a chair out from under the table and extravagantly straddled it. ‘So we’re going to keep you here pretty much as long as we like.’

‘You don’t have to tell me she was killed, you pompous bastard,’ Tom hissed, holding out a sleeve still flecked with Jennifer’s blood. ‘I was holding her hand when she died, remember?’

In a way he was glad that Stokes was acting like this. It gave him a reason to be angry, to give himself over to his rage, to feel its intoxicating opiate course through his veins and his pulse quicken. Better that than allow his sadness to envelop him, feel the paralysing arms of grief tighten around him as he subjected himself to a Sisyphean analysis of what he could and should have done to save her.