(Reuters) - Last week, Brazilian investigators accused four of the world’s largest oil-trading firms of participating in a corruption scheme focused on Brazil’s state-controlled oil company Petrobras.

FILE PHOTO: The logo of commodities trader Glencore is pictured in front of the company's headquarters in Baar, Switzerland, July 18, 2017. To match Insight BRAZIL-CORRUPTION/OIL-TRADERS REUTERS/Arnd Wiegmann/File Photo

Federal prosecutors allege Vitol, Glencore, Trafigura and Mercuria along with several small firms collectively paid at least $31 million in bribes over a six-year period to Petrobras insiders to secure oil deals at advantageous prices. Prosecutors said what they have uncovered so far is the “tip of the iceberg”.

The investigation is ongoing and the firms have not been charged. Mercuria denied the allegations. Vitol, Mercuria and Glencore said they would cooperate with the Brazilian investigation while Trafigura said it was reviewing the allegations.

Trafigura, Mercuria and Vitol said they have a zero-tolerance policy for bribery and corruption. Glencore said it takes ethics and compliance seriously.

Arrest warrants were issued for 11 suspects, eight of whom are in custody. None were current employees of Vitol, Glencore, Trafigura or Mercuria.

The four firms move 10 percent of the world’s daily oil consumption and have combined revenues of nearly $630 billion. But despite their size, they are not household names.

Here is a look at the companies and some details from last week’s law enforcement action in Brazil.

THE TRADERS

Historically, these companies have kept low profiles, making profits off of transporting commodities like copper, coal and crude oil from the mines and fields to consumers. With a high appetite for risk, they have worked in conflict areas and with controversial governments.

As their traded volumes have grown over the last ten years, they have invested globally in strategic pipelines, ports, major fuel station networks, shipping, large oil refineries, mining and even some oilfields.

Trafigura and Glencore are both successors to a firm run by the late Marc Rich, the man who invented oil trading.

Belgian-born Rich was indicted by U.S. authorities in 1983 on a host of criminal charges including tax evasion, wire fraud and trading with Iran in violation of U.S. sanctions. Rich was pardoned by U.S. President Bill Clinton.

Glencore, Switzerland-based and listed in London, bought miner Xstrata in 2013 and is now among the top four biggest listed mining companies. The firm remains the third-biggest oil trader, but its energy business is a small part of its overall portfolio. It also has an agricultural trading business.

Along with energy, Trafigura has a major metals and minerals division that accounted for more than half of its gross profits this year. Mercuria has also developed a metals and minerals team and has invested in oil production.

London-based Vitol has stayed energy-focused and is the world’s largest independent oil trader at over 7 million barrels per day.

Prosecutors alleged that smaller oil trading firms were also involved in the suspected bribery scheme.

(GRAPHIC: Volumes and revenues of the four oil traders - tmsnrt.rs/2zWgcLA)