Who killed Ashraf Marwan?

The billionaire's body tumbled over the railing of his apartment's fourth-floor balcony and landed hard on the London sidewalk. And like so much in the complicated life of Ashraf Marwan - a 62-year-old Egyptian who had been the most effective spy in the history of the Middle East - the mysterious circumstances of his death two weeks ago provoked further speculation.

As Scotland Yard investigates the suspicious fall, and as newspapers and bloggers throughout the world wonder whether any of several intelligence services played a role in his death, a debate continues over whether Marwan was a well-connected and resourceful Israeli spy or a brilliantly manipulative Egyptian double agent.

Marwan's death has also brought a new and chilling significance to a long-running legal battle in Israel involving the unauthorized leaking of his name to journalists. And in the aftermath of the discovery of his broken body on a sidewalk in the St. James neighborhood on June 27, I cannot help but wonder if I had a small part in the events that led to Ashraf Marwan's death.

Marwan's story - a tale overflowing with the suspense and ruthless duplicity of a spy novel - began to take shape in the spring of 1969.

He had come to London, ostensibly to consult a Harley Street doctor about a stomach ailment. He chose to be examined by a doctor whose offices had been used previously for a covert meeting between King Hussein of Jordan and the general director of the Israeli prime minister's office.

Along with his X-rays, Marwan handed the doctor a file crammed with official Egyptian state documents. He wanted them delivered to the Israeli Embassy in London.

The Mossad, the Israeli intelligence service, determined the documents to be genuine. Still, a rapidly formed working group of Mossad wise men debated the risk in dealing with a walk-in, a volunteer who shows up bearing gifts.

If he's not a double - an agent spreading disinformation - then he's uncontrollable. It was decided, however, that this walk-in's credentials were worth the gamble.

Marwan, the excited vetters discovered, was married to a daughter of Egypt's president, Gamal Abdel Nasser. He was also Nasser's liaison to the intelligence services. Not even 30, he was an intimate of the leaders who determined Egypt's future.

Three days after meeting with the doctor, Marwan was contacted by the Mossad as he walked through Harrods, the London department store.

His operational life as a spy began.

From the start, Marwan delivered. He yielded so many top secret Egyptian documents it was as if, as one Mossad agent put it, "we had someone sleeping in Nasser's bed." Based on this trove of secrets, Israel developed what became an article of faith for the nation's political and military leaders: "the Concept." With biblical certainty, the Concept held that until 1) Egypt possessed missiles and long-range bombers and 2) the Arab states united in a genuine coalition, a new war with Israel would not take place.

Running the agent, who was given code names including "Angel," "Babylon" and most frequently "the In-Law," grew into a small industry. For face-to-face meetings with his handler and often the head of the Mossad, a safe house was purchased in London not far from the Dorchester Hotel. It was wired to record every conversation, every aside. A special team of clerks turned the tapes into transcripts for the prime minister, the army chief of staff and a handful of other top Israeli officials. Marwan received £50,000 at each meeting, but this was only a minor expense compared to the estimated $20 million spent over the first four years of Marwan's operational life.

Israel's leaders felt this was money well spent: They knew what their enemies were thinking.

Then in April 1973, the In-Law sent a flash message to his case agent using the word "radish." This was the code for an imminent war. Zvi Zamir, the head of the Mossad, rushed from Tel Aviv to the London safe house. The In-Law revealed that on May 15, Egypt and Syria would launch a surprise attack.

Israel called up tens of thousands of reservists and deployed additional brigades and support equipment in the Sinai and the north.

The alert dragged on for three months and cost $35 million.

But it was a false alarm. The In-Law had been wrong.

Six months later, on Oct. 5, 1973, the In-Law sent another flash message with the code word "radish." Zamir was awoken at 2:30 a.m. with the news. The next morning, he took the first El Al flight to London.

Syria was massing tanks and missiles in the north. Egypt was conducting military maneuvers near the Suez Canal. Russia had begun evacuating families from the region. Yet that afternoon General Eli Zeira, the head of Israeli military intelligence, announced at a staff meeting that a coordinated attack by Egypt and Syria was "low probability - even lower than low."

Just before midnight, London time, the In-Law appeared at the safe house. He spoke to Zamir for less than an hour and then left.

Zamir phoned an aide at 3:40 a.m. on the morning of Yom Kippur, the holiest day on the Jewish religious calendar. The Egyptians and Syrians, he said, will attack simultaneously on both fronts at sunset.

At an Israeli cabinet meeting that morning, the In-Law's warning was not considered persuasive. The last time he had promised war would break out, nothing happened except the expenditure of $35 million.

Moshe Dayan, the minister of defense, lectured the army chief of staff, "On the basis of messages from Zvika you do not mobilize a whole army."

Nevertheless, it was decided that at 4 p.m. - two hours before the In-Law said the attack would be launched - armored brigades would move into position along the Suez Canal. Until then, there would be only three tanks in position to hold off any invasion.

At 2 p.m., the Arab armies went to war. Egypt crossed the Suez Canal in the south and Syrian tanks charged from the north. Their armies overwhelmed the surprised and unprepared enemy. After three days of fighting, General Dayan worried openly about the "destruction of the third Temple," the state of Israel. Prime Minister Golda Meir was given a bottle of suicide pills; she preferred to die rather than witness the destruction of the Jewish state.

Israel's outnumbered forces fought back and recovered their key positions. After being rearmed by airlifts of weapons and supplies from the United States, they attacked. Before the month's end, Israel won the war.

Still, the Yom Kippur War was an Israeli intelligence disaster.

Decades later, the Mossad and military intelligence continued to argue over who was to blame. Zeira, who lost both his job as head of military intelligence and a good deal of his reputation, spent years sifting through the events leading up to the attacks.

He wondered: Who had spread the false Concept? Who had "cried wolf" in May 1973 and persuaded Israel to call up its reserves? Who had been wrong about the time of the invasion? The answer, Zeira was certain, was that Israel had been deliberately and artfully misled.

From the start, the In-Law had been a double agent. The Mossad formed a special committee to examine the In-Law's role.

Its conclusion: Marwan was not a double.

But Zeira was unconvinced. He began to talk to journalists about his theory. I was one of those he spoke to. He never told me the spy's name, but he pointed me in a direction that made it easy - less than a half hour of searching the Internet - for me to deduce his identity. I used Marwan's name in a 2003 book about the Yom Kippur War.

Not long after its publication, Zvi Zamir called Zeira a "traitor" for divulging Marwan's identity. Zamir petitioned the attorney general for an investigation. But there was no official inquiry, and Zeira sued for slander. Last month, an Israeli Supreme Court justice ruled in arbitration that Zeira had in fact revealed Marwan's identity.

Now with his unexplained death, the many enigmas of Ashraf Marwan's complex life have grown even murkier. In Egypt, Gamal Mubarak, the president's son and possible successor, and Omar Suleiman, the head of the Egyptian intelligence service, attended Marwan's funeral. Sheik Mohammad Seyed Tantawi, Egypt's highest-ranking imam, led the prayers over the coffin, covered with an Egyptian flag.

On the following day, in response to reporters' questions, President Hosni Mubarak called Marwan "a patriot," according to Egypt's official Middle East News Agency. "He carried out patriotic acts which it is not yet time to reveal," the president added.

In Israel, an angry Zamir told the newspaper Haaretz, "I have no doubt that reports published about him in Israel caused his death." The former Mossad chief again called on the attorney general to indict Zeira.

In London, Marwan's sister was described as saying she saw him in good spirits only hours before his death. But another unidentified friend said Marwan, in declining health, lost his balance and fell.

And there were reports that he made many enemies through his activities in selling armaments. A coroner's inquest is expected to announce its findings in mid-August.

And now I am reminded of my last telephone conversation with Ashraf Marwan.

"Are you afraid?" I asked.

"Why should I be afraid?" he replied. "I was a soldier."

Marwan promised to reveal more about which country he was fighting for when we were to appear together on a news program in the United States. But two days before the taping, he called to tell me he would not speak in public until he had finished a book about the war.

I never heard from him again. Now Scotland Yard - and, I suspect, other agencies - is trying to find the manuscript he said was writing at the time of his death.

Howard Blum, a contributing editor at Vanity Fair, is the author of "The Eve of Destruction: The Untold Story of the Yom Kippur War."