Italian prosecutors investigating state-owned oil giant Eni SpA over a Nigeria oilfield deal three years ago believe half of the $1.1 billion that Eni paid went as bribes to government officials or to intermediaries with close ties to them.

Reuters said Thursday it had seen a letter from Italian law enforcement officials to the UK’s Crown Prosecution Service that asked for help freezing assets of Nigerian suspects.

Italian prosecutors said in the letter that “at least $533 million (about N86billion) was paid to Nigerian officials and intermediaries who helped secure the sale,” Reuters said.

The Milan prosecutors have placed Eni, its former chief executive Paolo Scaroni, and current CEO Claudio Descalzi under investigation for alleged corruption in connection with Eni’s 2011 purchase of a 50 percent interest in Nigeria’s OPL 245 offshore oil field.

Royal Dutch Shell holds the other 50 percent stake and hasn’t been implicated in the bribery probe.

The two individual defendants have denied any wrongdoing and haven’t been charged.

Eni said all the money for the purchase went to the Nigerian government and Malabu Oil and Gas.

Malabu reportedly had close ties to former Nigeria oil minister Dan Etete.

Eni denies any “illegal conduct” and said the money was not used to influence public officials or the purchase process.

A British court acting at the request of Milan prosecutors froze two bank accounts belonging to Emeka Obi that contained a combined $190 million.

Obi is thought to have served as an intermediary for the OPL 245 deal.

The deepwater acreage has estimated reserves of about 9 billion barrels of oil.

British police opened an investigation last year into the OPL 245 transaction.

Transparency campaigners in Nigeria had complained that Malabu Oil and Gas was registered just five days before it was awarded the OPL 245 block for just $2 million. It then flipped the block to the Eni / Shell venture for $1.09 billion.

“The proceeds of crime unit is investigating a money-laundering allegation in the UK in connection with OPL 245. The investigation is at an early stage, a UK spokesperson said in July 2013.