What do we really know about tax havens? In an extract from his new book, Nicholas Shaxson explains how they work and why they are so rife with secrecy, corruption and intimidation

In 2009 I met a former private banker, Beth Krall, to explore a question that had been nagging me: how do bankers who shelter the wealth of gangsters and corrupt politicians justify what they do? We met one Sunday in Washington DC. She had left private banking and joined the non-governmental sector. Dressed in a striking black-and-white coat, she still looked very much the stylish international financier. Aged 47, and with nearly 24 years in the banking business, Krall (not her real name) was still coming to terms with her past life.

Krall's last offshore posting was in the Bahamas, an island archipelago with over 300,000 residents that has been an important offshore centre since the golden age of American organised crime. A few months earlier, a practitioner in the Caymans had warned me to watch out for my personal safety if I went "asking all these questions" in the Bahamas. Krall said she was unsure what might happen to her if she went back, as she was partly breaking the private bankers' code of silence. "I don't want to have concrete shoes put on me," she said without smiling. One reason for her fear was something that had angered her in the first place: so many of the people she dealt with were powerful members of society in their home countries.

Krall took her banking exams straight after school, and then worked for a number of banks before moving to Cititrust in the Bahamas, where she ran evaluations and accounting for their mutual funds business.

From this point, Krall declined to name her employer. She became a client relationship manager with the private banking arm of a well-known international bank in the Bahamas. They worked with what are euphemistically known as managed banks or shell banks, an offshore speciality. These have no real presence where they are incorporated, so they can escape supervision by regulators.

The terrorist attacks on 11 September 2001 prompted the US to legislate against shell banks. A bank in the Bahamas must now employ two senior bankers and keep its books and records there to be judged real enough to do business. "That means a bank maybe with a room or suite in a building, with two people in it – that's a bank now," Krall said. She directed me to the website of a Bahamas-based trust company that will provide you with exactly that: the appearance of being a real bank – including two staff members as directors and a place to keep the books. Such a setup can allow business almost as usual, yet still tick the regulators' boxes.

Krall moved to a big European bank, again as a client relationship manager – in effect, someone who finds wealthy clients and keeps them happy. Trawling for business, she was routinely pointed towards Latin America, where she travelled frequently. "On the immigration form you would write that you were going for pleasure, though your suitcase would be full of business suits and portfolio evaluations, or marketing materials and presentations explaining the advantages of a trust in the Bahamas." The client's name didn't appear on their portfolio evaluation: in fact, the bank would not even record it as the account name. It was nerve-racking, sometimes, going through airports, but she always got through unchallenged.

Despite her growing qualms, Krall ended up working for a boutique Swiss private bank in the Bahamas. This was no ordinary bank, and was the only one where she actually saw a suitcase full of cash. "My bank never once had a client walk through the door," she said. "The bankers and their clients go on big-game hunting trips, or to the ballet in Budapest. That is where it happens."

Her colleagues hailed from old European aristocratic circles. While Krall was perfectly good at her job and had close working relationships with top lawyers, asset managers and so on, a gap remained. "They went to parties with royalty, with ambassadors," she said. "I wasn't in their circle."

At the time, laws in the Bahamas were being tightened a little, following a feeble global crackdown, and she moved sideways in the bank to work as a compliance officer. These days, offshore bankers make a big show of their know-your-customer rules to keep out the bad money. Depositors may have to supply a certified copy of a passport, for example, and divulge where their money came from. Jurisdictions such as the Bahamas and the Cayman Islands put these requirements into their statutes, and banks employ compliance officers such as Krall to enforce this. That, at least, is the theory. But there are many ways around the restrictions.

Krall was supposed to check for suspicious movements through the accounts – of which there were plenty. She raised many red flags. "They [her managers] would say, 'This was a commission'." Were these bribes? Commissions on what? "I went back, and never got an answer." One Swiss-based trust company that had a relationship with her bank displayed almost nothing on its website, bar some photos of a nice fountain in Geneva. "The crap they brought to us was unbelievable. There is no way a responsible trustee should take this on. You would have no idea who the trust settlors were, what the assets were or where they came from. I objected strongly, but the bank took them on."

There is something about island life that stifles dissent. In the island goldfish bowl, you cannot hide. The ability to sustain an establishment consensus and suppress troublemakers makes islands especially hospitable to offshore finance, reassuring international financiers that local establishments can be trusted not to allow democratic politics to interfere in the business of making money.

John Christensen, Jersey's former economic adviser-turned-dissident, describes encountering extremist right-wing offshore attitudes when he returned to his native island in 1986 after working overseas as a development economist. It was the year of the City of London's Big Bang of financial deregulation, and he found the tax haven amid a spectacular boom. Old houses, tourist gift shops and merchant stores in Jersey's beautiful capital St Helier were being knocked down and replaced by banks, office blocks, car parks and wine bars. He went to an employment agency and they told him he could have any job he wanted. The following day he had three offers. In his work he soon became aware of practices such as reinvoicing, in which trading partners agree on a price for a deal, then record it officially at a different price in order to shift money secretly across borders.

As the river of money flowing into Jersey became a tide, he expressed unease about the origins of some of it, much of it from Africa, but he was brushed aside.

The concentration of extremist attitudes in Jersey was self-reinforcing, as Christensen explains. "Most liberal people like myself left," he said. "My socially liberal friends from school, almost all of them left Jersey to go to university, and almost all of them didn't go back. I can't tell you how dark it felt." He almost left, but was persuaded to stay by academic researcher Mark Hampton, who was putting together a framework for understanding tax havens and convinced him how important it was to understand the system from the inside. "I went undercover," Christensen said, "not to dish the dirt on individuals and companies, but because I couldn't understand it – and none of the academics I spoke to could either. There was no useful literature."

Jersey is riddled with elite, secretive insider networks, typically linked to the financial sector. After being appointed economic adviser in 1987, Christensen found that many people who came to see him wanted him to join their Masonic lodge, and gave him the secret signal. "Their thinking is very much of the old-boy network – you are either one of us or you are against us," he continued. "It means they can trust you to do the right thing without having to be told – an insidious meaning of the word 'trust'."

Unaccountable elites are always irresponsible, and I got my own flavour of Jersey's mouldy governance on the first day of a visit in March 2009, when the Jersey Evening Post carried a front-page story headlined "States in shambles", referring to the States Assembly, Jersey's parliament. "The States resembled a school playground yesterday as foul language and personal insults flew across the chamber," it said. Senator Stuart Syvret, a popular but controversial politician, had complained in the assembly that the health minister was whispering in his ear.

Syvret, the newspaper reported, stood up and said: "On a point of order, I am sorry to interrupt the minister. But the minister to my right, Senator Perchard, is saying in my ear: 'You are full of fucking shit, why don't you go and top yourself, you bastard.'" Senator Perchard responded by saying: "I absolutely refute that. I am just fed up with this man making allegations." The BBC, which was broadcasting the sitting live, had to apologise for the language.

Syvret has been a regular victim of efforts to suppress dissent. "Any anti-establishment figure here is bugged," said Syvret. "There is a climate of fear. Anyone who dares disagree is anti-Jersey, an enemy of Jersey. You are a traitor, disloyal. There is all this Stalinist propaganda." A few weeks after my visit eight police officers arrested Syvret and held him for seven hours while they ransacked his home and personal files, including his computer.

In October 2009, having been accused of leaking a police report about the conduct of a nurse, Syvret fled to London and claimed asylum at the House of Commons, saying he could not get a fair trial in Jersey. British Liberal Democrat MP John Hemming put Syvret up in his flat, declaring that "we should not allow him to be extradited, to be prosecuted in a kangaroo court". When Syvret returned in May 2010 to fight an election he was arrested at the airport. "This is a society with no checks and balances, run by an oligarchy," Syvret said. "It is a one-party state, and it has been for centuries."

At the Smugglers' Inn on Jersey's beautiful coast, I sat with John Heys, a tour guide at the world-famous Durrell zoo, and his friend Maurice Merhet, a retired printer and pig farmer. The two had spoken out – in letters to the Jersey Evening Post and in other forums – and have been decried, publicly and regularly, as traitors. Both described the same climate of fear that Syvret had: the dread of being squeezed out of a job, of never getting anywhere, of being blacklisted.

Heys showed me an email from a government minister to a dissident friend who had, in a cheeky Christmas message to the minister, pointed out the large sums stashed away in Jersey amid global poverty. The minister responded – mistakes included: "Hi Traitor, Please refrain from sending me your unsolicited garbage … I am surprised you still decide to live in this 'tax haven' island … ifs its so bad why do you not leave to live somewhere else … good riddance I would say … but perhaps NOT because you get a damm good living here, no doubt perhaps funded by banks and your morgage lender … in fact my family have lived in Jersey for several generations and I am so very proud of it but to listen to traiterous idiots like you makes me furious. I would not have the nerve to wish you a happy christmas in fact I hope you continue to live a miserable existence in your traiterous world."

One night in 1996, towards the end of his time in Jersey, Christensen opened the books for a reporter from the Wall Street Journal, who was investigating an alleged fraud ring involving American investors and a Swiss bank operating out of Jersey. The story, headlined "Offshore hazard: Isle of Jersey proves less than a haven to currency investors", ran on the front page several months later. Jersey's finance industry and politicians went into spasm. This was one of the first times Jersey's supposedly clean and well-regulated finance sector had been challenged in a serious global newspaper. The end of the article quoted a senior civil servant. Everyone in Jersey was sure it was Christensen. He knew that, in talking to the reporter, he had effectively resigned.

Finance can take advantage of insularity, timidity and moral shortsightedness, but the ethos of the Jersey establishment derives ultimately from the offshore industries and their onshore controllers, not from innate island character. Offshore repression can happen in larger jurisdictions, too. Rudolf Elmer, a Swiss banker who had worked for banks in several offshore centres before becoming a whistle-blower on some of the corruption he had seen, felt the pressure in Switzerland, a country of eight million people.

In 2004 Elmer noticed two men following him to work. Later, he saw them outside his daughter's kindergarten, then from his kitchen window. His wife was followed in her car. The men offered his daughter chocolates in the street and late at night drove a car at high speed into the cul-de-sac where he lived. The stalking continued, on and off, for more than two years. The police said there was nothing they could do. In 2005, they searched his house using a prosecutor's warrant, and he was imprisoned for 30 days, accused of violating Swiss bank secrecy, which is, as he put it, "an official violation, like murder".

"I was thinking of suicide at this stage," he said. "I would be looking out of the window at 2am. They intimidated my wife, children and neighbours. I was an outlaw. I was godfather to a child whose father is in finance. He said I have to stop – 'you are a threat to the family'." A relative was pressured at work to avoid contact with Elmer; after one warning he left the office in tears. "I was bloody naive to think that Swiss justice was different," Elmer said. "I can see how they might control a population of 80,000 people in the Isle of Man, but eight million? How can a minority in the banking world manipulate the opinion of an entire country? What is this? The mafia? This is how it works. Jersey, the Cayman Islands, Switzerland: this whole bloody system is corrupt."

This is an edited extract from Treasure Islands: Tax Havens and the Men Who Stole the World by Nicholas Shaxson, published by The Bodley Head this week, price £14.99. To order a copy for £11.99 with free UK p&p go to theguardian.com/bookshop or call 0330 333 6846.

For more information see treasureislands.org