Nigerian ex-attorney general Mohammed Adoke has been arrested in Dubai, his lawyer told Reuters on Wednesday, after a warrant was issued earlier this year as part of one of the oil industry’s biggest corruption scandals.

However the lawyer, Mike Ozekhome, said the arrest had been based on an expired warrant.

Nigeria’s anti-graft agency secured warrants in April for two former ministers including Adoke and an Eni manager over the $1.3 billion sale of a Nigerian offshore oilfield by Malabu Oil and Gas in 2011.

The deal has spawned legal cases spanning several countries, involving Nigerian government officials and senior executives from ENI and Royal Dutch Shell. Shell and Eni, and their executives, have denied any wrongdoing.

Adoke was arrested on Monday, Ozekhome told Reuters, as his client went to Dubai for medical appointment.

He added however that a Nigerian court had nullified the warrant in October because his client was not served with the charges before the warrant was issued.