Via del Governo Vecchio, Rome 19th March-11.32 a.m.

The streets were dark and narrow here, the buildings seeming to arch together over Tom and Allegra’s heads like trees kissing over a country lane. It was busy too; people carefully picking their way along the narrow pavements, dodging around the occasional dog turds and an elderly woman who was furiously scrubbing her marble doorstep. The traffic, meanwhile, was backed up behind a florist’s van which had stopped to make a delivery. Alerted by the relentless sounding of impatient car horns, a few people were leaning curiously over their balconies, some observing events with a detached familiarity, others hurling insults at the van driver for his selfishness. Glancing up, he made an obscene gesture, and pulled away.

Allegra was silent, her eyes rarely lifting from her shoes. She was hurting, Tom knew, probably even blaming herself for Aurelio’s betrayal, as if his selfishness and pride was somehow her fault. He tried to think of something to say that might comfort her and relieve her imagined guilt. But he couldn’t. Not without lying. The truth was that in time the floodwaters of her anger and confusion would recede, leaving behind them the tidemark of their lost friendship. And whatever he said, that would never fade. He, of all people, bore the fears of betrayal.

‘What other Phidias pieces are there?’ he asked, stepping to one side to let a woman past holding on to five yapping dogs, the leashes stretching from her hands like tentacles.

‘There’s a torso of Athena in the École des Beaux-Arts in Paris that’s been attributed to him,’ she replied without looking up. ‘And they found a cup inscribed with his name in the ruins of the workshop at Olympia where he assembled the statue of Zeus.’

‘But nothing like the mask?’

‘Not even close.’ She shook her head. ‘If Aurelio’s right, it’s priceless.’

‘Everything has a price,’ Tom smiled. ‘The trick is finding someone willing to pay it.’

‘Maybe that’s what Cavalli was doing the night he was killed,’ she said, grimacing as an ancient Vespa laboured past, its wheezing engine making the windows around them rattle under the strain. ‘Meeting a buyer. Or at least someone he thought was a buyer.’

‘It would explain why he had the Polaroid on him,’ Tom agreed. ‘And why he hid it when he realised what they really wanted.’

‘But not where he got the mask from in the first place.’ She paused, frowning, as the road brought them out on to the Piazza Ponte Sant’Angelo. ‘What are we doing here?’

‘Isn’t this where you said Cavalli was killed?’ Tom asked.

‘Yes, but…’

‘I thought we should take a look.’

A steady two-way traffic of pedestrians was streaming over the bridge’s polished cobbles, the hands and faces of the statues lining the parapet seeming strangely animated under the sun’s flickering caress, as if they were waving them forward. For Tom, at least, the wide-open vista was a welcome relief from the narrow street’s dark embrace.

‘Where did they find him?’ he asked, hands shoved deep into his coat pockets.

‘In the river. Hanging from one of the statues.’

‘Killed on the anniversary of Caesar’s murder, only for Ricci to be murdered on the site of Caesar’s assassination,’ he said thoughtfully.

‘With both Ricci’s and Argento’s deaths staged as a re-enactment of a Caravaggio painting.’ She nodded impatiently. ‘We’ve been through all this.’

‘I know.’ He shrugged. ‘It’s just that everything about these murders has been so deliberate. The dates, the locations, the arrangement of the bodies, the careful echoing of some element of the one that had preceded it. It’s almost as if…they weren’t just killings.’

‘Then what were they?’

Tom paused before answering. In the distance the glorious dome of St Peter’s rose into the sky, massive and immutable. Around it swarmed a flock of pigeons, their solid mass wheeling and circling like a shroud caught in the wind.

‘Messages,’ he said eventually. ‘Maybe someone was trying to have a conversation.’

‘If you’re right it started with Cavalli,’ she said slowly, her eyes narrowing in understanding.

‘Exactly. So why kill him here? Why this bridge? They must have chosen it for a reason.’

Allegra paused a few moments before answering, her face creased in thought.

‘It was originally built to connect the city to Hadrian’s mausoleum. Before becoming a toll road for pilgrims who wanted to reach St Peter’s. And in the sixteenth and seventeenth centuries, famously of course, they used to display the bodies of executed prisoners along it as a warning.’

‘A warning to who?’ Tom frowned, then nodded at the weathered shapes looming over them. ‘What about the statues? Do they mean anything?’

‘Commissioned from Bernini by Pope Clement IX. Each angel is holding an object from the Passion. Cavalli’s rope was tied to the one holding a cross.’

‘Which was then echoed by Ricci’s inverted crucifixion and Argento being found in a church.’ Tom clicked his fingers as two more small pieces of the puzzle fell into place.

‘That’s not the only thing,’ Allegra added excitedly, a thought having just occurred to her. ‘Cavalli’s not the first person to have been killed here.’

‘What do you mean?’

‘A noblewoman called Beatrice Cenci was tortured and put to death on the Piazza Ponte Sant’Angelo in 1599,’ she explained. ‘It was one of Rome’s most notorious public executions.’

‘What had she done?’

‘Murdered her father.’

Tom nodded slowly, remembering the deliberate violence with which Cavalli’s house had been ransacked.

‘Patricide. Treason. Maybe that’s it. Maybe Cavalli had betrayed the League and this was his punishment?’ He gave a deep sigh, then turned to her with a shrug. ‘Your guess is as good as mine. Come on, let’s try and call Archie. He should have landed by now.’

They turned and walked to the end of the bridge, Tom reaching for his phone as they waited for a break in the traffic. But before they could cross, a large armoured truck gunned down the road towards them. Two men jumped down holding what Tom recognized as what the Sicilian mafia called a Lupara-a traditional break-open design shotgun, sawn off a few inches beyond the stock to make it more effective at close range and easier to manoeuvre and hide. The weapon of choice in old-school vendettas.

A woman behind screamed and Tom could hear the fumbling scramble of panicked feet behind him as people scattered.

‘Get in,’ one of the men barked.

Lungotevere Vaticano, Rome 19th March-11.53 a.m.

Looking around him, Tom could see that the truck’s interior had been furnished like an expensive office, the floor laid with thick carpet, the sides lined with a cream wallpaper decorated with tropical birds. To his left a red leather sofa abutted what he assumed was a toilet cubicle, its door latched shut. In the far right-hand corner, meanwhile, stood an elegant cherrywood desk on which a brass banker’s lamp illuminated a laptop and a police scanner spitting static. Overhead were four flat-panel screens, each tuned to a different news or business channel. Most telling, perhaps, was the gun rack opposite the sofa, which contained four MP5s, half a dozen Glock 17s and a pair of Remington 1100s. Neatly stacked on the shelves below were two dozen grenades and several boxes of ammunition. Enough to start and win a small war.

The gears crunched and the truck swayed forward with a determined snarl. The gunman who had followed them inside waved at them to sit down and then instructed them to handcuff themselves to the hoop bolted to the wall above them so that their arms were held above their heads. Stepping forward, he made sure that the ratchets were tight against their wrists and then emptied their pockets and Tom’s bag, pausing over the FBI file and the Polaroid of the ivory mask. In the background, Tom could make out the opening aria of the Cavalleria Rusticana.