Nigeria’s former attorney general was detained by the country’s financial crimes agency,EFCC, upon his return to Nigeria on Thursday, as part of an investigation into one of the oil industry’s biggest suspected corruption scandals.

Mohammed Adoke was arrested by Interpol in November after travelling to Dubai for a medical appointment. He voluntarily flew back to Nigeria on Thursday, his lawyer said.

The Economic and Financial Crimes Commission (EFCC) investigation relates to the $1.3 billion sale of a Nigerian offshore oilfield known as OPL 245 by Malabu Oil & Gas in 2011.

Eni and Shell jointly acquired the field from Malabu, which was owned by former petroleum minister Dan Etete.