Milan prosecutors have placed the CEO of Italian oil and gas company Eni under investigation over alleged bribes paid for a lucrative Nigerian oil concession.

Eni confirmed Thursday that Claudio Descalzi and another official were under investigation but denied wrongdoing, saying the investigation would confirm their innocence. It pledged to cooperate with investigators.

The purported bribe concerns Eni's purchase of the lucrative OPL 245 oil block in 2011. Eni and Shell insist they paid the Nigerian government directly for the concession; prosecutors are investigating whether the money went to a front company believed controlled by a former Nigerian oil minister.