Charles McGonigal, a retired top FBI counterintelligence agent who led the Trump-Russia probe, was arrested on Wednesday after it was discovered he had been working for a sanctioned Russian oligarch.

Prosecutors allege that McGonigal, a British citizen, had been helping the Russian oligarch, Oleg Deripaska, dodge US sanctions. Spanish police arrested McGonigal in connection with the case.

McGonigal, who was a former FBI official, is facing charges of violating Russian sanctions. He is also accused of working for Deripaska in an effort to influence US policy.

The arrest of McGonigal has raised questions about the integrity of the Trump-Russia investigation, which he had led. It has also caused concern among US officials, who are now worried that the Russian oligarch may have been able to influence US policy.

The case is being closely watched by US officials, who are eager to find out more about McGonigal’s ties to the Russian oligarch.