A mass suicide turned Robert Balch into an instant media star. Tim Cornwell hears how a Montana sociology professor infiltrated the notorious Heaven's Gate cult all in the name of research.

A bombshell had landed on Robert Balch's doorstep, but he did not realise it for several hours. On the Wednesday night police discovered 39 bodies in a mansion outside San Diego, California, the phone began to ring. "I got calls saying 'that sounds like the group you studied', and I was saying 'no, I don't think so'," he says. First reports claimed - inaccurately - that the dead were mostly young white males. And the name, Heaven's Gate, was strange to him.

But while helicopters hovered over the home of the UFO cult, Balch, a University of Montana sociology professor, emerged as the chief academic source on America's largest mass suicide. Not only had he studied the group, he had carried his research to the point of enlisting as a member in the 1970s.

America was thirsty for news. Mid-Thursday Balch stopped answering his phone. On Friday evening, in the thick of 40 press interviews, he asked a friend to go through his voice-mail. "It took her four hours," he says. He had become a celebrity overnight.

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When Balch collected his PhD in sociology, his speciality was juvenile corrections. But it was cult leader and former opera singer Marshall Herff Applewhite who set the path for his academic career. In 1975, Applewhite and mental nurse Bonnie Lu Nettles, known then as "the Two", created a media sensation when they promised to lead 30 followers to a rendezvous with an alien spacecraft. Balch went to a meeting and was fascinated. Twenty-two years later, Applewhite's death would again turn his life upside down.

Balch believes in hands-on research. He does not just lecture on "extraordinary group behaviour" among cult movements: he takes his students to see it. The principle is "let's find out what their point of view is", but it has led him down some strange avenues. He spent spring with a group of Montana students at a white separatist religious compound called Elohim City, in Oklahoma, now infamous as the place to which Timothy McVeigh placed several phone calls in the days before he allegedly killed 168 people in the Oklahoma City bombing, fuelling theories of a wider conspiracy. The students had "a great time", because the residents of Elohim City were so hospitable, he says, but the trip earned criticism that it legitimised racists.

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In 1995 Balch contributed a chapter to The Gods Have Landed: New Religions from Other Worlds. In it, he describes his encounters with the Heaven's Gate cult. In 1975, stumbling across a meeting in Prescott, Arizona, he watched 25 members present what they called "the message". The cult had no name as such. But Applewhite and Nettles, known as "Bo" and "Peep", "Do" and "Ti", after the musical notes, and even "Guinea" and "Pig", had convinced their followers they were soon to leave earth on a spaceship. He considered asking if he could come along as a sociologist to document their final days on the planet. Instead he decided to pose as a recruit.

He and a Montana student, David Taylor, joined the group for two months. They wrote their notes in the bathroom stalls. Balch flanked speakers at cult meetings as a "buffer" against negative thought-waves. He helped post flyers for meetings on UFOs. On a leave of absence, he ended up roaming southern California with three other cult members, begging for petrol and food. They eventually ended up at a remote rendezvous, a "cold, wind-swept expanse of desert with broken-down cars and almost nothing to eat".

Balch is ambivalent about the rights and wrongs of cults, and of Heaven's Gate in particular. Sociologists argue bitterly about free choice in these "new religions", but he does not describe its members as victims. They joined voluntarily, and doubters were not welcomed. "Seekers had to want membership in the next kingdom more than anything else. Those who had to be persuaded obviously weren't ready to leave the planet." His writing earned him a note from Applewhite thanking him for "trying to maintain my objectivity".

But once in, Balch says, Applewhite's followers were given guidelines designed to break off attachments to the "human level", and build an almost military commitment to the group. They were forbidden to read newspapers, watch television or call their parents. Sex and friendship were discouraged. In what was known as "tomb time", members spent days without talking.

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Through the 1970s, the UFO cult dwindled to a few dozen members. In the early 1980s, Balch stopped tracking the group because it had become so secretive. But in 1994, soon after the cult, then called "Total Overcomers Anonymous", placed newspaper adverts making its "final offer" to earth's inhabitants, two members turned up in his office. They were making a documentary on the cult and wanted to interview Balch.

Cults, says Balch, have been a feature of the American scene since the Utopian societies started in the early 1800s. His current research focuses on a group in Missoula called the Bahai's Under the Provision of the Covenant, a "classic millennium group" under a leader who has visions of the approaching apocalypse. At one point, fearing a nuclear terrorist attack on New York City, the sect built up a network of bomb shelters.

He was "stunned" when he realised Heaven's Gate was the group he had tracked, not least because he knew several people who died. He is still puzzled as to when the idea of a suicide took hold. Applewhite's original writings made it clear, he says, that members would take their "vehicles" - bodies - with them when they left the earth, meaning suicide "just didn't fit". But that may have changed when nurse Nettles died of cancer in 1985. "That was not part of the game plan, and caused them to rethink how they might be leaving the planet," he says. Cult members talked of rejoining "Ti" on the spaceship following the Hale-Bopp comet. He plans to link up again with Taylor, his former student, to fill in the gaps about the past ten years. "There are so many unanswered questions," he says.