The mastermind behind JPMorgan Chase’s “London Whale” trades has been arrested in Spain.

Former JPMorgan trader Javier Martin-Artajo was among two men charged by US prosectors of trying to hide a complex trade at the bank that led to more than $6 billion in losses.

According to reports, Martin-Artajo handed himself over to authorities in Madrid — but he insisted he would fight extradition to the US, where he faces up to 20 years in prison.