A judge has placed the head of a Mexican oil services firm under house arrest under suspicion that his company defrauded US bank Citigroup in a multi-million-dollar case. Amado Yanez, chief executive and majority shareholder of Oceanografia, will be held for up to 40 days as prosecutors determine what charges might be lodged against him, Attorney General Jesus Murillo Karam told a news conference. Citigroup said late last month that it was slashing $235 million from its 2013 financial results due to the fraud committed against its Mexican subsidiary, Banamex. The US banking group said $400 million of $585 million in credit given to Oceanografia by Banamex was based on bogus invoices from the energy firm, which is a major supplier of Mexico's state-run oil monopoly Pemex. The scandal prompted the Mexican government to take control of Oceanografia, a company that employs 11,000 people and has a fleet of around 70 ships. The government has also taken over a first division football team owned by Yanez, the White Roosters of Queretaro, as part of the investigation.