The integrity of a Bank of England inquiry is in question as prosecutors in the US probe what the central bank knew about traders’ behaviour in the foreign-exchange market.

The Department of Justice has secretly requested an interview with a senior Royal Bank of Scotland forex trader previously questioned as part of the BoE’s own investigation into whether any of its officials knew of or connived in manipulation of forex, one of the world’s biggest financial markets. US prosecutors are concerned over the thoroughness of the BoE’s report and the way its inquiry was handled, according to people familiar with the situation.

The DoJ request — made without informing the UK authorities — comes at a sensitive time for the bank. It is trying to recast its image as a more transparent and accountable institution, even as it has been drawn into an unprecedented criminal investigation by the UK Serious Fraud Office. The SFO is separately looking at the extent of any BoE officials’ involvement in the alleged manipulation of money-market auctions at the onset of the financial crisis.