The US Department of Justice is allegedly widening its probe into allegations that some of the world's biggest banks have sought to manipulate the foreign exchange market.

The DoJ is understood to be questioning a range of FX professionals over their sales practices.

According to sources cited by Bloomberg, DoJ authorities have started to collect data from bankers about how much they charge customers for exchanging currency for the first time, in a bid to root through relevant sales practices.

US authorities have ramped up their investigations in allegations of FX fixing over the last few months and in April stormed a set of London banks to grill foreign exchange traders that are potentially involved with rigging currency rates.

A source told IBTimes UK at the time that the DoJ and the UK's Financial Conduct Authority conducted joint interviews with London-based individuals at un-named banks, over allegations of rate rigging.

In October 2013, the FCA confirmed it is probing the foreign currency market amid reports that a number of regulators are investigating allegations of market manipulation by some of the world's biggest banks.

One month later, the regulator said it is mulling over creating a new set of rules for the largely unregulated currency markets after the completion of a raft of foreign exchange manipulation investigations.

Meanwhile, UK Chancellor George Osborne pledged to impose tougher criminal sanctions on crooked bankers who manipulate financial markets.

Legislation after the Libor-fixing scandal to bring in a maximum seven year jail term for interest-rate riggers will be extended to those caught fiddling with rates in the currency, commodities and fixed income markets.

The FX Market

The daily $5tn (£3.1tn, €3.7tn) currency market is the largest in the financial system and is pegged to the value of funds, derivatives and products.

Morningstar estimates that $3.6tn in funds, including pension and savings accounts, track global indexes.

FX rates are calculated and compiled by using data from a variety of submitted provisions on a number of platforms, such as ThomsonReuters.

It is then calculated by WM, a unit of State Street, to form WM/ThomsonReuters at 1600 GMT daily.