Mexico’s attorney general has ordered the arrest of the chief executive of oil services company Oceanografia on charges related to a fraud probe involving Citigroup’s local unit Banamex.

Oceanografia boss Amado Yanez is under police protection in hospital after undergoing emergency surgery on 20 May, according to a Reuters report citing the attorney general’s office.

Yanez, who is also majority owner of Oceanografia, would be formally placed under arrest once he had left hospital, a spokesman for the attorney general’s office told the news wire. It was not clear why the 49-year-old Yanez had surgery.

The attorney general’s office held Yanez for questioning in March after Citigroup said it uncovered at least $400 million in bogus loans to Oceanografia at Banamex. He has been accused of participating in crimes including violations of Mexico’s credit-institutions law.

“This may be the government showing that it’s cleaning up the energy sector ahead of the oil opening,” Javier Oliva, a political and security analyst at Mexico’s National Autonomous University, told Bloomberg.

Oceanografia was a top service provider to state-run oil giant Pemex over the past decade, winning billions of dollars in contracts. Some 97% of Oceanografia’s revenues come from state-controlled Pemex, according to Bloomberg. It was siezed by the Mexican government in February.

The services company was banned in February from bidding on government contracts for 21 months due to alleged violations of agreements with Pemex.

That ban prompted Citigroup to review its loans with Oceanografia, and determined that only $185 million of the collateral backing $585 million of loans could be verified, Bloomberg reported.

Citigroup has discovered some $565 million in loans linked to fraud at Banamex and earlier this month the bank fired 11 employees amid an internal review.

Banamex faces a US criminal investigation of possible violations of money-laundering laws, a source has told Reuters, as well as probes by US and Mexican authorities into the fraudulent loans.