Italian prosecutors are investigating Royal Dutch Shell’s involvement in a Nigerian oil deal, drawing the oil company into a corruption probe over the acquisition of an offshore field in the African country in 2011. The probe that has been ongoing since 2014 has dogged Italy’s energy giant Eni and its CEO, Claudio Descalzi, who both have denied any wrongdoing, Reuters reported.

The prosecutors are investigating whether Shell’s piece of a $1.3b payment to acquire a rich oil field off the coast of Nigeria constituted a bribe. Shell and Eni jointly owned a Nigerian license, known as OPL 245 developing giant Atlantic Ocean oil fields, thought to contain 9b barrels of oil. It has been a substantial project for the companies in the country.

Italian and Dutch police raided Shell’s headquarters in The Hague in February looking for evidence that could be used in the case, The Wall Street Journal reported. Currently, Italian prosecutors are working jointly with an anti-fraud team in the Netherlands in order to determine whether the two oil companies paid bribes to obtain the licences for the Nigerian site.