He rose from humble working class origins to become the shipping world's leading magnate by following his gut instincts and taking huge risks.

Secretive Norwegian billionaire John Fredriksen, known as Big Wolf and more widely as Big John in the shipping industry, came under scrutiny this week when U.S. regulators launched one of the biggest ever crackdowns on oil price manipulation.

Fredriksen, 67, who has rivalled earlier tanker tycoons such as Greece's Aristotle Onassis, is now embroiled in a case in which two of his trading firms are accused of making $50 million by squeezing markets in 2008.

He is a rough businessman, said editor Gunnar Stavrum of Norway's Nettavisen, who has written two unauthorized biographies about Fredriksen.

He has been involved in speculative trading before, among other things in currencies. It's too early to say if these claims of illegal oil trading will hold up.

Fredriksen began his career as a messenger boy at a shipping company in Oslo, later establishing himself on the shipping scene during the 1960s and 1970s in Singapore and New York.

He made his fortune in the tanker wars of the 1980s during the Iran-Iraq conflict, when his vessels risked missile fire to load and then transport crude oil cargoes from the conflict area.

ESTIMATED $10.7 BILLION FORTUNE

He is not someone who likes to follow rules, said a ship industry source.

Fredriksen by his very nature is a massive chancer, he's not an analyst or an academic kind of guy -- he does things by gut feel which is no different from any other big players in the past like Onassis.

Fredriksen's aide Tor Olav Troeim was not immediately available for comment on Wednesday. Players in the industry when contacted were hesitant to offer views on Fredriksen, who has long shunned the media.

He is the biggest ship owner in the world and has built up everything from scratch. Impressive is my only comment, said one oil tanker source.

Forbes magazine has named Fredriksen as among the world's 100 richest billionaires with an estimated fortune of $10.7 billion. Norwegian business magazine Kapital puts his fortune at a similar level.

Fredriksen is a widower with two daughters, 27-year old twins, who have increasingly become involved in the running of his businesses in recent years.

John just turned 67 the other day, but he says he hasn't made enough money to retire, Troeim said earlier this week.

The magnate, an avid soccer and fly-fishing fan, consolidated his position in shipping through Oslo-listed Frontline, the world's largest independent oil tanker company, which he controls. He has also taken a large stake in the lucrative Norwegian fish farming industry.

TANKER BOOM

Frontline enjoyed the boom in freight rates over the past few years which saw average earnings on the benchmark Middle East Gulf to Japan route rocket up to $180,000 a day before economic turmoil in 2008.

Average earnings have slumped to as low as under $400 a day this year, prompting Frontline to announce on Wednesday it may divest assets as the sector struggles with a prolonged downturn and mounting fleet growth.

Fredriksen's vessel the Sea Empress went down on the rocks off Milford Haven on Britain's west coast in 1996, in one of the country's worst environmental disasters.

In the mid-1980s, he was briefly held by Norwegian police investigating the disappearance of oil from some of his crude carriers, which it later emerged was used as fuel. The practice was seen as unsafe, and the case was later settled with a fine.

In recent years, Fredriksen has been seen as a leader in oil tanker safety, introducing double hulls and other measures to avoid oil spills.

Fredriksen's energy empire also includes liquefied natural gas company Golar and leading offshore driller Seadrill as well as a controlling stake in dry bulk group Golden Ocean. He also controls the world's biggest fish farmer Marine Harvest.

But it's two of Fredriksen's energy trading companies that have caught the attention of U.S. regulators.

The Commodity Futures Trading Commission (CFTC) said on Tuesday traders James Dyer of Oklahoma's Parnon Energy, and Nick Wildgoose of Europe-based Arcadia Energy, amassed large physical positions at a key U.S. trading hub to create the impression of tight supplies that would boost oil prices.

Later they dumped those barrels back onto the market, causing prices to crash and racking up profits from short positions they had accrued in futures markets, the suit said.

Both companies are controlled by Fredriksen's Farahead Holdings, based in Cyprus. Arcadia rejected on Wednesday the CFTC claims saying it was wrong on both the facts and the law.

Like anyone who runs VLCCs (super tankers), oil is their lifeblood and he fancied himself as a bit of an oil trader which is why he bought Arcadia. It's still a small enterprise, said the ship industry source.

While he took Cypriot citizenship in 2006, Fredriksen runs his businesses from London. According to the websites of companies he controls, he established a series of holding companies indirectly controlled by trusts established for the benefit of his immediate family.

Many of his companies are registered in Bermuda, with operations located in offices around the world, including Norway.

Fredriksen has also been a vocal activist shareholder in German travel group TUI in which he owns a minority stake and has been fighting for a seat on the board for years. Before patching up differences this year he had accused other TUI stakeholders for their support of what he called incompetent management.

(Additional reporting by Michele Kambas in Nicosia, writing by Jonathan Saul, editing by Anthony Barker)

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