In January 2002 it was reported on the Irish news that a woman's body had been found in a rented house in Donnybrook, Dublin. Her name was Rosemary Toole and, police said, she had been suffering from depression. Her suicide would probably have gone unreported were it not for the fact that she'd been spotted at Dublin airport a day earlier, picking up two jolly-seeming Americans at arrivals. The three of them were then seen drinking Jack Daniels and coke at the Atlantic Coast Hotel in County Mayo. At one point - other drinkers later testified to the police - Toole stood up to go to the toilet and did a jig at the table. The next day she was dead and that night the two mysterious Americans, one wearing a dog collar, left Dublin.

The Irish police released the names of their suspects. They were seeking the arrest and extradition of the Reverend George Exoo and his partner Thomas McGurrin, of Beckley, West Virginia, for the crime of assisting a suicide, which, under Irish law, carries a maximum prison sentence of 14 years.

I telephoned Exoo to ask if I could follow him around. I imagined myself as being pro-assisted suicide, although I didn't know much about the ins and outs. But it seemed only fair to let someone kill themselves and have a reverend at their side if that is what they wanted. The Irish prosecutors struck me as draconian and anachronistic. I wasn't alone in believing this: radio phone-in shows across Ireland were ablaze with callers supporting Toole's right to kill herself with a reverend at her side.

And so, at dawn on a Monday in 2003, Exoo and I set off in his clapped-out old Mercedes towards Baltimore (a five-hour drive) to visit a new prospective client, Pam Acre, who said she had been suffering from chronic fatigue syndrome since the 70s and was considering killing herself later in the year. Exoo was paying for the petrol even though he was broke. He said he asked for donations from his clients but often didn't get them, but he didn't care because this was his calling.

"I've never done anything as important as this in my ministry," he told me en route. "I think it's the reason I was placed on this planet. I'm a midwife to the dying, for those who want to hasten their deaths."

Exoo was cheerful, quite giggly, a gay, liberal, libertarian Unitarian preacher, cultured, funny, charming. He said he often carried around a large, gas-filled inflatable alligator to his "exits" in case the police stopped him on the way. He often used gas as a suicide method. With the alligator, he could pretend he was a children's party entertainer. But lately he had begun phasing the alligator out.

"It made me feel conspicuous," he said. "Part of the thing is that I want to not be noticed. I'm always careful and I always work quietly, like the Lone Ranger. I do so generally at night and for the most part I make it look like they just died in their sleep. I'll prop a book up on their lap so it looks like they just expired. But if I'm carrying a big alligator people are going to notice me."

"Plus," I said, "surely the last thing your clients would want to see in the minutes before death is a large inflatable alligator coming through the door."

"Exactly," said Exoo.

So that was why he no longer kept an alligator in the boot of his car.

There was something Laurel and Hardy-ish about Exoo. Earlier he had demonstrated the gas method for me by attaching a hose to the end of a gas tank, but he did something wrong in the preparation and the gas tank practically exploded, shooting the hose across the room and whiplashing his stomach. We all shrieked.

"Does this happen when you're helping people?" I asked, aghast.

"This has never happened before," Exoo replied, a bit sheepishly.

Acre lived in a decrepit old country cottage in the outskirts of Baltimore. She looked as crumbling as her house. She was 59 but she acted 30 years older. She let us in. We all sat on her sofa.

"Tell me about your illness," Exoo asked her.

"This is a difficult disease to cope with," Acre replied, "because they run all the tests and they come back negative. Then they decide that ..."

"It's all in your head," said George.

"Right," said Pam.

They smiled at each other.

"They start wanting you to go to psychiatrists," said Acre. "But of course that's totally useless."

"Sure," said Exoo, softly. "Sure."

Exoo wasn't, I noticed to my surprise, saying anything to Acre that might possibly dissuade her from committing suicide. Instead they talked about the "mechanics of the dying" (what pills and gas and apparatus Acre would need) and she seemed delighted to have someone there who didn't question her symptoms or intentions at all. Then she turned to me.

"I've learned what I can from this," she said, indicating her skinny, frail body. "I don't judge much of anybody for anything. Because until you walk in somebody else's shoes you do not know."

Exoo told me he drifted into assisting suicides in the early 90s when he was a Unitarian minister in Pittsburgh. Unitarianism is a middle-class, liberal religion and Pittsburgh is a tough, working-class town, so he didn't have many parishioners. He would look at his tiny congregation and wonder if he was wasting his life.

One day a parishioner approached him and said, "My husband has got ALS [amyotrophic lateral sclerosis, a form of motor neurone disease] and your name has been given to me as someone who might help."

"It was that vague," Exoo said. "But I knew what she meant. Two weeks later he said to his wife, 'It's time. Call George Exoo.'"

That's how Exoo found his calling. He says he has gone on to assist 102 people, including Acre, who killed herself, with Exoo at her side, a few months after our visit.

In early 2004, Irish police formally instigated extradition proceedings against Exoo. They asked the FBI to arrest him. Exoo telephoned me. Could I come to Seattle? He had something he wanted to tell me. It was all quite cloak and dagger. I met him in the lobby of a Seattle airport hotel. There was a slightly weird smile on his face.

"What is it?" I asked him.

"I've ordered a magic potion because I certainly don't intend to travel to Dublin," he replied. "So I may be the first right-to-die martyr."

There was a silence. Exoo gave me a big, profound look. I didn't know what to say. It was a bit of an awkward moment.

"Other than that," I said, "how are you?"

"Fine," he said. "So, anyway, maybe I should call you over to Beckley for the big event."

I narrowed my eyes. He wanted to kill himself and he wanted me on hand to chronicle it.

"I don't want to," I said.

He looked a bit disappointed.

Exoo was in Seattle for a private meeting of international right-to-die activists. The biggest names in the movement were here, such as Derek Humphry, a former British Sunday Times journalist who wrote a bestselling memoir, Jean's Way, about helping his terminally ill wife commit suicide in 1975. Jean's Way pretty much began the movement: a network of right-to-die groups inspired by it sprang up across the world in the late 70s. These activists meet once a year in an anonymous hotel somewhere to discuss advances in suicide technologies.

"It's very hush-hush," Exoo said. "I'm surprised they're letting you in."

The delegates sat around a table in a conference room. Exoo began by announcing his intention to martyr himself rather than face extradition. Then he scanned the room. I think he was expecting an outpouring of shock and sympathy but in fact people seemed much more interested in what method he was intending to use.

"My curiosity is why would you go with a drug approach?" one delegate asked him, and then the others leaned forward, paying attention.

Exoo's reply was that, when one uses gas, the person killing themselves often tries to involuntarily remove the apparatus once they're unconscious, and he has to hold their hands down, and he "didn't want to involve anyone else in my passing".

He then changed the subject, saying that Toole in Dublin had promised to send him a message from beyond the grave. The message would somehow take the form of roses. And she fulfilled the promise the day after she died.

"What happened was Thomas [Exoo's partner] and I flew out the next morning to Amsterdam," he said, "and a man brushed by us on the street. He had roses flung over his shoulder. I've never seen anybody with so many roses. There must have been 10 dozen roses! And Thomas said, 'There she is! There she is!'"

There was a silence. Then Dr Pieter Admiraal, a pioneering Dutch advocate of euthanasia, coughed grumpily and said, "Oh, dear George. To meet somebody with roses in the Netherlands is not so extreme because we are growing them to export to the whole world."

There was laughter from the others.

"And now you are in trouble," Admiraal added. "Maybe God can help you."

"Maybe so," snapped Exoo.

That evening I approached Admiraal for an interview. We talked for a while about Exoo's idealism.

"He's too good for this world," Admiraal said. Then he added, "I've been observing him for a long time, and I've asked our psychiatrists to observe him ..."

He paused. He was obviously weighing up whether to tell me whatever was on his mind.

"He is," he finally said, "in my opinion, enjoying the death of another person. And that's dangerous. I have the strong impression that he wants to be there and see something dying. Well, he cannot help that. It's his character. It's a kind of phobia to enjoy death. And that's why he says, 'I will commit suicide.' Because he will want to die at that moment."

(Later, Admiraal clarified this. He said he didn't mean Exoo derived psychopathic pleasure from being around death. Instead, he thought Exoo was too in love with the afterlife. He believed in it too much and the pleasure he got was from clapping and cheering his clients to this better place.)

I didn't know what to think. I hoped Admiraal was being overdramatic because I had considered myself pro-assisted suicide, and once your mind is set on something it can be hard to do a 180-degree turn. Still, my own doubts had been creeping in. For instance, I'd noticed that very few of Exoo's clients were terminally ill. Most were depressed or suffering from psychosomatic diseases. When I asked him about his client list he said, "Many of my colleagues will avoid such persons like the plague but I feel a very strong identity with the story of the good samaritan. I stop while others walk by and ignore their pleas."

How, I wondered, did Exoo and his clients find each other?

After the conference, I visited Humphry, father of the modern right-to-die movement. He is from Wiltshire but now lives in Oregon.

"Once or twice a week," Humphry explained, "I get very strange people on the telephone who are anxious to commit suicide because of their depression or sad lives. When they get your number they want to talk and talk. And they call again and again. And they also call all the other right-to-die groups."

Humphry said that the mainstream right-to-die groups will tell them, "'We can't help you. It's not within our parameters because you aren't terminally ill.' But they pursue you. They call and call. And eventually someone will say, 'George Exoo will probably help you.' And that gets them off the phone and on to George."

"Isn't that terrible?" I asked.

"Oh, yes," said Humphry.

Three years passed. Even though the Irish government was pressing the FBI to arrest Exoo, they didn't. Meanwhile he was travelling around America helping non-terminally ill people die.

In the spring of 2007 a package arrived. It was a video. I went into my office, closed the door, put it into the VCR and pressed play. Exoo's messy office blurred fuzzily on to the screen. Then he skulked into shot from behind the camera, looking as if he had been awake for days.

He said, "Now, what I'm going to do is call my friend Shirley who is out in a western state in a motel."

And he picked up the phone and dialled and it became obvious what was going to happen. A woman called Shirley was sitting in a motel room in Arizona waiting for his call. She had a bottle of poison in front of her and he was going to guide her through her suicide.

He said, "Hey, Shirley. This is George. The hour has come that we've been planning."

He hadn't bugged the phone so I could only hear his end of the conversation.

"I know you're nervous," Exoo said. "You've never done this before. But that's all right. We're going to get through this. It's time for you to" - he sighed - "drink the potion that's in front of you. It's bitter and horrible tasting so it's important that you chugalug it right down. I ask you to raise that glass and I want you to know how honoured I am to be with you at this moment."

There was a silence of perhaps 10 seconds. Then Exoo's voice hardened: "I know it's bitter. Just keep drinking. Put your finger over your nose and get it all down."

He was talking to Shirley like someone would talk to a child who had disobeyed him. Then he began to chant a Buddhist chant: "Gate, Gate, Parasamgate ... "

(Gone. Gone. Gone completely beyond.)

After five minutes he said, "Shirley? Can you hear me?"

Then he looked into his camera. He said, "I think I heard the phone drop. Which would mean she is probably now gone."

He shrugged slightly. "And that's it. That's the way it's done."

He leaned over and turned off the camera.

Exoo's irritable attitude towards Shirley was startling but it wasn't totally unexpected. He had, he told me, behaved in a similar manner towards Acre. She too had prevaricated, and he told her he wasn't leaving town until he had "finished" guiding her through her suicide. I'm sure Exoo wasn't encouraging reluctant clients to kill themselves - I'm sure the choice was always theirs - but he did seem to speak impatiently at them during apparent moments of hesitation.

In May 2007 Exoo began teaching a friend, Susan (not her real name), the ropes. He said he needed an assistant in case he was arrested or killed himself. We arranged to meet at Susan's house in North Carolina. I arrived before Exoo. Susan lived alone, a middle-aged lady with a collection of plastic lizards. While we waited I asked her how they met.

"I was bitten by a brown recluse spider in 1993," she replied. "It was so painful I wanted to die."

She said she called the official right-to-die groups, "but they wouldn't help me."

"Because you weren't terminally ill?"

"Yeah, they rejected me," she said. "But then somebody said, 'You might want to call George.' Kind of like under the counter."

Susan said she would have killed herself with Exoo's help - he was perfectly willing - but she couldn't find anyone to look after her pet snake. Eventually, they got talking. If she wasn't going to be his client, perhaps she should be his assistant.

Then Exoo arrived. He had a second job now, buying up houses that had been seized by the banks, and then selling them on for a quick profit, although he hadn't managed to sell any yet. It was oddly nice to see him. We joked about how he could provide his clients with the full service: he could sell them a house and when the banks foreclosed he could help them kill themselves. For all the ambiguities, he was fun to be around. And he was a strange mix, often going to the ends of the Earth to help people in distress, but also getting cross with them if they seemed hesitant about killing themselves.

I said to him, "In the Arizona tape, Shirley said, 'It's bitter,' and you snapped, 'Drink it!'

"Absolutely," he replied. "Because I'd been through that argument with her before."

"She'd tasted it before?" I asked.

"Yeah," he said. He was getting annoyed with me. "I'd been with her twice before in person. What kind of bull twaddle is that? If you're serious you're going to drink it and not whine about it!"

"But this was somebody who didn't know whether to kill themselves," I said.

"Just drink it," he said. "Three or four swallows and you're going to go to sleep. Permanently. In 10 minutes you'll be off this planet. Yes, I was probably pressing her to some extent. But I was pressing her to make up her mind one way or another because I can't go flying across the country week after week and have nothing come of it. I want her to either go on and live her life, or check out. But it's her choice. It's not mine."

We went for lunch. Susan had told me that morning that her multiple chemical sensitivities (triggered by the 1993 spider bite) were so severe there was only one local restaurant she could eat in where the atmosphere did not set off her symptoms. But we ate in another restaurant - an all-you-can-eat buffet - and she was fine. She ate all she could. I began to see Susan as living proof that Exoo really shouldn't help people like Susan kill themselves.

After lunch I told him about the view - held by some psychologists who had observed him - that he was out of his depth and it was a slippery slope and he should stop.

"And what's the slippery slope I'm on?" he asked sharply.

"Not being able to stop helping people because you see it as your calling and you like to be there at the moment of death because you get something out of it. And you may consequently be encouraging them towards suicide."

"Bullshit," he said. "It just hasn't happened. Otherwise these people wouldn't be hanging on for years and years and years."

And that part seemed to be true: he claimed to have clients who had been prevaricating for years.

Exoo drove off to do some real estate business and I was left alone with Susan. We sat on her porch. And she said something extraordinary. She said that unbeknown to Exoo she had set up her own suicide business and was willing to help practically anyone if the price was right.

"I see this as a business," she said. "George sees it as a calling. There's a big difference there. For me it's no cash, no help." She said her price was approximately $7,000.

"You're bound to get it wrong, aren't you?" I said. "And help someone who shouldn't be helped."

Susan shrugged. "Probably, at some point, yes," she said.

She said Exoo's worst crime was his financial imprudence: that he'll help people who can't afford to pay.

"George will get to a point where he'll run out of money," she said. "He won't scale down the expensive cuts of meats every night. He would rather kill himself than economise."

"He seems quite keen on killing himself," I said.

"I think he'll do it soon," said Susan. "And that's why I've been pressing him to give me a list of his current clients."

A few weeks passed. Then I got an early morning call from Susan. She said the FBI had just arrested Exoo. His partner McGurrin had woken up to find Exoo and two men standing there. They said, "We're putting George in prison until we can take him to Ireland."

A few weeks after that (I later learned) Susan flew to New Zealand to help a depressed, non-terminally ill woman she had met on the internet commit suicide. The woman had previously asked a mainstream right-to-die group called Dignity NZ to help her, but they had refused.

"I was of the impression that she needed assistance in living rather than advice on how to end her life," Dignity NZ's founder, Lesley Martin, later explained to me in an email. She added, "I imagine you are developing a good understanding of what an absolute mess the euthanasia underground is. Unfortunately, there are 'gung-ho' individuals involved [she meant Susan] who, in my opinion, treat the matter of assisting someone to die as an exciting relief from the boredom of their own lives and do so completely ill-equipped and dismissive of the responsibility we have of ensuring that people who need mental health assistance receive it, while still working towards humane legislation that addresses the real issues."

I visited Susan and asked her what had been wrong with the New Zealand woman. "She had some sort of breathing disorder," she said, "and the doctors there wouldn't give her the medication that she needed. I happened to take the same medication. I gave her a little bit of mine and she was fine."

"But you helped her commit suicide, even though you helped her breathe better?" I asked.

"Yeah," said Susan. "Isn't that ironic?"

"You shouldn't do it," I said.

"Somebody's got to pay the bills so you can have some water in that glass you're drinking," she said.

I had agreed to protect Susan's identity before I knew she was going around the world helping people die for money.

On October 25 2007 a federal judge in Charleston, West Virginia, freed Exoo. He decreed that because assisted suicide is not a crime in 25 of the 50 states, he couldn't allow the Irish prosecutor to try him in Dublin.

I visited Exoo one last time. I thought there wouldn't be any more twists and turns in this story about the messy assisted suicide underground, but he offered one.

"You know I provided you with a tape?" He meant the Shirley/Arizona telephone tape. "That was not a real deathing. I was talking to a dial tone."

I looked at him askance.

"You're a very good actor," I said.

"I wanted to give you an example of how I would work with somebody," he shrugged. "And she was the only possibility."

He explained that Shirley was a real person, and he really had visited her on many occasions, and that she really had prevaricated. All that was true.

"She really is now dead," he added, sounding quite triumphant. He said she killed herself in Kingman, Arizona, while he was in prison. (The Kingman police later confirmed this.)

I think Exoo was stupid to fake that tape and that someone who helps people decide whether or not to kill themselves shouldn't play weird games like that. The things I liked most about Exoo when I first met him six years ago - his libertarian, maverick qualities - are actually the things that are most worrying about him.

· Jon Ronson's documentary, Reverend Death, is on Channel 4 on May 19 at 10pm.