U.K. prosecutors opened a criminal investigation into alleged manipulation of foreign-exchange benchmarks, more than a year after the British markets regulator first began a review.

The Serious Fraud Office is probing “allegations of fraudulent conduct in the foreign-exchange market,” the London-based agency said in an e-mailed statement.

Authorities around the world have been investigating whether traders rigged the $5.3 trillion-a-day currency market after the Financial Conduct Authority opened its review last year. Regulators and prosecutors are scrutinizing allegations that dealers at the world’s biggest banks traded ahead of their clients and colluded to rig the WM/Reuters rate, a benchmark that pension funds and money managers use to determine what they pay for foreign currencies.

More than 25 traders have been fired, suspended or put on leave after the allegations emerged last year. Banks including Citigroup Inc., Deutsche Bank AG, Barclays Plc and UBS AG, the top four currency dealers according to Euromoney Institutional Investor Plc’s annual survey, have said they are cooperating with the probes.

Spokesmen for the banks declined to comment on the SFO investigation.

DOJ Investigation

While the FCA has taken the lead in probing the allegations in the U.K., the U.S. Department of Justice has been investigating possible criminal angles to the matter since last year. Prosecutors there could bring charges and levy fines in the case as soon as this year, a person with knowledge of the affair said last month.

The SFO’s decision to wait to open an investigation of its own has prompted criticism from British lawmakers, who said the alleged offenses were similar to those found in manipulation of the London interbank offered rate, or Libor.

The Bank of England has also come under criticism from lawmakers in the currency-rigging affair. It suspended one employee and engaged an outside lawyer, Anthony Grabiner, to probe allegations central-bank officials condoned practices at the heart of the investigations.

Grabiner has been contacting traders who attended a 2012 meeting of a BOE committee, according to two people with knowledge of the matter. The chief dealers’ subgroup would meet with central bank representatives a few times a year to discuss trends and issues in the market. Grabiner is looking into allegations that the officials condoned practices such as sharing impending customer orders with counterparts at other firms.

The SFO is separately prosecuting a dozen people in London over the manipulation of Libor. Sixteen individuals are facing charges worldwide for alleged rigging of the interest-rate benchmark.