The disappearance had taken place 18 years earlier. What he wanted was to trace witnesses to the disappearance, or people in the area at the time.

He said he wanted me to fly to New Guinea and do face-to-face interviews. There was, he said, a lot of money behind him. He could make arrangements to pay me what my business earned for the time I was away. "If you learn what has happened to the person who has disappeared, there'll be substantial rewards."

He saw me hesitating, leaned closer and whispered: "Fifty thousand."

He snapped open an attache case and put the down payment of $US4000 in crisp bills on my desk. Then he had me sign a contract. As I was doing so, Gross dropped his little bombshell.

"The person who has disappeared is Michael Rockefeller, heir to one of the biggest fortunes in the world. He went missing in 1961. My client is Mrs. Rockefeller, first wife of the recently deceased Nelson."

Just as it is now, New Guinea in 1979 was split into two countries. Papua New Guinea, the eastern side of the island, had been administered by Australia since the end of World War II.

The area in which Rockefeller had disappeared was in the other half; in the south of Irian Jaya, to be precise. It was a long way away from civilisation. At the time of his disappearance, the Dutch controlled the territory. The native and headhunter people, the Asmats, who lived in the muddy delta where Rockefeller had gone missing, had no political aspirations. They just wanted to be left alone to hunt heads.

By the time Gross made his visit to my office in 1979, Irian Jaya was under the control of the Indonesian military. Much of the region was wetlands - impenetrable swamps and jungle populated by headhunters and cannibals. The area where Rockefeller had gone missing was in the south and was as much under control of crocodiles as of the military.

Soon I learned that Michael's mother, Mary, who had divorced Nelson Rockefeller in 1961, had been actively prevented from finding her son by her former husband. He had feared it would impede his political career. Upon Nelson's death, Mary had immediately set out to learn as much as she could about Michael's disappearance. Gross had come to see me within days of Nelson's passing. Now Mary was finally free to find out what had really happened to her son.

In November 1961, 23-year-old Michael was in the treacherous region where the Asmat people lived, the wetlands off the Casuarina Coast, West Papua. He was collecting artefacts to display in the New York Metropolitan Museum of Art and had indicated to other anthropologists and friends that he was also hopeful of setting up a museum for the West Papuans themselves.

It was Rockefeller's second visit that year. Earlier, he'd been part of a Harvard Peabody Museum anthropological expedition to film native tribes untouched by westerners.

Rockefeller and a friend linked up with two Dutch anthropologists, Adri Gerbrands and Rene Wassing, who had already spent some time there. Wassing was assigned by the Dutch administration to keep an eye on the billionaire's kid and make sure he didn't come to harm in Dutch territory, but was little more than a yes man, quietly going along with whatever Rockefeller wanted to do.

Rockefeller had to leave the Asmat region in July, but he determined to return as soon as possible with Wassing. By late September 1961, he was flying back to New Guinea, apparently to buy shields, canoes and heads for his museum. He linked up again with Wassing and made his way back to the Asmat. This was headhunter territory. Rockefeller was starting to feel that here was the place where he could make his mark in history. He tried to make contact with a tribe from a nearby place called Otsjanep, who ever the most recalcitrant of all the Asmats. They hunted heads, they sucked brains, they butchered women and children. And they weren't stupid.

Their favourite pastimes had been outlawed by the Dutch police and frowned on by the missionaries, but they knew well enough how to conceal the fact that they were still practicing centuries-old customs. It was here among these people that Rockefeller spent his last weeks, in the jungle buying skulls and carvings.

The official story of Rockefeller's last days goes like this. On Saturday, November 18, Rockefeller and Wassing were in the town of Agats on the coast preparing to sail south. Despite advice to take the rivers, Rockefeller was determined to take the quicker sea route. His vessel, a catamaran, was really two canoes with a central bamboo platform and an outboard motor.

Two Asmat guides accompanied Rockefeller and Wassing. At the Agats pier, police inspector Henri Watrim saw how low in the water the boat was sitting and ordered Rockefeller to lighten its load. Rockefeller did as he was told - until Watrim had gone on his way. Then he reloaded and the laden catamaran sailed southeast into the open sea.

The party put in at the village of Per a while later so that Rockefeller could inspect a canoe which he was going to purchase. They left at 2pm and hit a squall. The boat was swamped, the outboard engine died. The guides abandoned the boat and Rockefeller and Wassing stayed. It took the guides five hours to get to shore. According to Wassing, the catamaran capsized and he and Rockefeller drifted around, clinging to the hulls until dawn. Around 8am, Rockefeller decided he should try to make the shore alone.

At 4pm on Sunday, November 19, a Dutch Neptune patrol plane spied Wassing on the upturned cat about 60k to sea off the Cook Bay area. Wassing was dropped supplies and finally rescued at 9am on Monday, November 20. There was no sign of Michael Rockefeller.

When Nelson Rockefeller learned of the disaster, he chartered a seaplane that would take him, Michael's twin sister Mary Strawbridge, their staff and dozens of international journalists to the island of Biak off the north coast of Irian Jaya. The Dutch sent its air force and navy to search for Michael and its administrators and police joined in too. The US sent an aircraft carrier and Australia a Catalina seaplane and two helicopters.

Most of the search was concentrated on the coastline, where it was expected that he might have been washed up. The search was finally called off. The consensus was that there was no hope for Michael. He had drowned or been eaten by sharks.