Royal Dutch Shell says Italian officials are investigating its role in the purchase of an offshore oil block in Nigeria for more than $1 billion.

Shell and the Italian oil firm Eni bought the block known as OPL 245 in 2011. Activists including the watchdog group Global Witness say the money ultimately went to a company secretly owned by a former Nigerian oil minister and never reached state coffers.

Global Witness and other groups filed a complaint in 2015 with the public prosecutor in Milan that the group says describes Shell's role in the transaction.

Eni was already being investigated. Both firms have denied wrongdoing.

In a statement this week, Shell said officials searched its headquarters in The Hague in February and that it was cooperating with officials.