NEW YORK — Criminal prosecutors from the U.S. Department of Justice have gone to London to interview individuals in connection with an investigation into traders' alleged rigging of foreign exchange rates, a person familiar with the matter said.

The DOJ has conducted joint interviews of UK-based currency traders with the United Kingdom's Financial Conduct Authority in London, the person said.

The London interviews highlight the extent to which U.S. and UK authorities are cooperating in a sprawling investigation into potential manipulation in the foreign exchange market, after clashing in conducting probes into the rigging of the Libor benchmark interest rate.

The FCA sought in 2012 to interview traders ahead of DOJ in ways that had the potential to complicate U.S. criminal investigations.

While the FCA could compel people to answer its questions and not remain silent, the U.S. Constitution allows individuals to remain silent under oath in order to avoid self-incrimination.

Any evidenced gathered under compelled conditions could be difficult for U.S. authorities to use, former prosecutors have said.

The recent joint interviews on the forex probe suggest the two authorities are now working in tandem.

After extracting billions in fines from global banks for rigging Libor, the average rate at which a panel of banks expects to borrow money, authorities have turned to other benchmarks, including those that undergird the $5.3 trillion-a-day currency market, to investigate whether they are open to similar kinds of skewing.

Banks including Barclays, UBS AG,, Royal Bank of Scotland, and now Rabobank have paid out billions in fines, and have agreed to turn over all information that the Justice Department asks of them for at least two years as part of their settlements, Mythili Raman, the acting head of the department's criminal division, said in October after the forex probe was announced.

In February Raman told Reuters the Justice Department had worked hard to improve its relationships with UK authorities.

A spokesperson for the DOJ was unavailable for comment on Sunday.