I wondered if I should tell the truth, but I excused myself with the thought that it would defile Felix’s memory without doing me any good.

“Nothing important,” I lied. “He seemed okay.”

That might have been it but for Gabriel, who was waiting for me when I left my office for lunch three days later. He sat on the sofa by the elevators, under a notice board on which some guides to mood disorders were pinned. He drew my attention because he was lounging easily-not like an anxious patient or a parent who was waiting for a child in treatment-and because I vaguely recognized him. As I walked by, I saw him scan my badge and look at my face, appraising me with narrow eyes. Then he got to his feet.

“Dr. Cowper? My name is Gabriel Cardoso. We had a friend in common, I think. Felix Lustgarten.”

He spoke unhurriedly, in a rich voice with an accent I wasn’t sure about-it sounded Spanish. Gabriel, that’s it. I remembered him standing on the balcony of his TriBeCa apartment at his party on the night of Greene’s death. I’d been talking to Lucia before we’d left together, and she’d pointed him out. I recalled his air of detachment, as if he didn’t know most of the guests but enjoyed having them fill the place.

“I was saddened by his death,” he went on. “We were not close friends, I would say, but we were once colleagues. He was a man I liked.” He gave the impression that he didn’t say it lightly: he had standards.

“I’m sorry, too. I’d just gotten to know him. Shall we?”

I gestured at the sofa and took a seat-I didn’t know how long I would want to stay. Gabriel reached into a jacket pocket and pulled out an envelope with his name and, I assumed, address scrawled on the front. Inside were two sheets of paper, well thumbed, and a small metal block: a computer flash drive.

“I got this in the mail yesterday,” he said, frowning. “It’s a letter from Felix, and he’d enclosed this.” He held the drive between thumb and finger. “There are a bunch of documents on it. I looked at them last night and found them interesting. Disturbing, in fact. Felix asked me to show them to you. Just you, no one else.”

I looked at the drive, now nestling in Gabriel’s hand. Felix had given no indication of having this in mind, and I couldn’t understand why he had sent an emissary from beyond the grave.

“If he wanted me to see them, why did he send them to you?” I said.

“Ah, well … They require some explanation.”

It dawned on me then. When Felix had talked of Greene trying to hide the losses in the Elements, he’d said they were hard to grasp. He’d sent Gabriel to help me, I realized. I was touched by his posthumous gesture: he hadn’t just written me off after I’d walked out on him. Yet it worried me to be entrusted with this legacy.

“You’re a rocket scientist?” I said, remembering Felix’s words.

“I am indeed,” he said, beaming. “Do you have some time to talk? Maybe somewhere private. I will need a computer for this.”

I hesitated for a few seconds, but I didn’t really have a choice. I owed it to Felix in death, no matter what he’d done in life.

I could have wasted hours in Gabriel’s apartment just looking around. Maybe that’s how he spent his time, since he seemed to have plenty to spare. In the sunlight, with the view of Manhattan I’d seen from his balcony only at night, it was captivating. It was long and wide, with a dovetailing maze of rooms into which light spilled from high windows. A couple of rooms seemed devoted entirely to art, with blinds drawn to protect his collection of drawings. There was no sign that he shared it with anyone: it was just him in his monument to Wall Street wealth.

“You have an unusual name, Mr. Cardoso,” I said, making small talk as he slotted the flash drive into a computer in his study and tapped at the keyboard, manipulating a baffling array of numbers.

“It’s Portuguese,” he said, smiling. “I am originally from Brazil, you see. I came here to teach mathematics at NYU. Wall Street head-hunters kept calling me. Trading is all mathematics now, based on models. Traders don’t understand it properly, so they need people like me. Rocket scientists, like you said. Most traders don’t have a clue what they’re doing.”

He sounded pleasantly amused by the idea rather than outraged. I began to realize why he looked so bemused by his surroundings and his wealth. They had been handed to him through a twist of educational fate.

“You worked at Seligman?”

“Used to. I was pushed out last year by Marcus Greene, before Harry killed him.” He chuckled as if Harry had meted out retribution for him. “I wasn’t in Greene’s clique, eh? And I said some things he didn’t like, too loudly. But I was there long enough to be comfortable,” he said, waving to his apartment. “That’s how I know Felix, and Lauren Faulkner as well. Felix mentioned her, I think?”

I didn’t reply, but my silence didn’t seem to bother him because he kept talking as if he hadn’t noticed.

“You want to know how we became friends? I had a nice office off the floor at Seligman. Just a glass box, but I had a very comfortable chair in there, a leather armchair. You couldn’t see who was in it from outside, and Lauren would come by to take a nap. They work stupid hours, bankers. They don’t get enough sleep.”

“That’s funny,” I said. I tried to imagine the ever-alert Lauren curled up on Gabriel’s chair. It would have been a more relaxing time, I imagined, when she hadn’t been under such strain. It made her seem human-not the woman who’d threatened me.

“I remember us talking one evening, before the merger. She said she was looking at the Grayridge books. I told her to be very careful. Things were becoming difficult in the markets and I’d heard rumors. It was very complex stuff, you know. I offered to take a look with her, make sure it was all okay. I had the feeling something might not be right. A couple of days later, she was gone.”

“Didn’t you find that odd?”

“No one gets much warning on Wall Street, you know. They put a trash bag on your desk for your stuff and escort you from the building in case you steal something. It’s like you’ve been executed. The same thing happened to me. I didn’t know how bad things had been at Grayridge until I saw this.”

“These are the Elements?” I said, looking over his shoulder at the screen. I could see what Felix had meant: it was just a blur of numbers to me.

“This is Radon. Let me explain.”

Grayridge had sold the Elements through the Cayman Islands to investors trying to make money from the housing boom before it fell apart, Gabriel said. The mortgages had been bundled together and then divided into securities with differing amounts of risk in them. It wasn’t even that simple, because they were synthetic CDOs, built out of credit default swaps based on mortgages. They’d been a crazy mash-up of financial risk.

“There were nine deals like this one structured by the CDO desk,” Gabriel said. “They did the same thing each time. Sold the equity and mezzanine to hedge funds and kept hold of the triple-A tranches. They issued the risky paper, the bonds that paid the most, to other investors. But the yield was very low on the super-senior tranches they kept, so they levered them up to make a return. They held about $120 billion, yielding $160 million a year.”

His explanation reminded me of what Harry had told me on the beach at East Hampton and in Riverhead, and I hadn’t understood that, either. I wasn’t treating Gabriel for depression, though, so I could ask him what I wanted.

“A hundred and twenty billion dollars?”

“That’s right. What’s your question, Ben?”

“It sounds like a lot of money.”

“It was, but the paper was supposed to be risk-free, like a government bond. There wasn’t going to be a default even if the mortgage holders stopped paying. They’d built it to take a hundred-year storm. Take a look here. This is the loss on all of the Element deals at the date of the merger. What does it say?” he asked.