The mother of Nigeria's Finance Minister Ngozi Okonjo-Iweala has been released five days after she was kidnapped, Delta state governor Emmanuel Uduaghan has told the BBC.He said Kamene Okonjo, a retired professor, was dropped off on a main road near her home in southern Nigeria.Kidnapping is common in southern Nigeria, where it is a lucrative criminal enterprise.It is not clear if Mrs Okonjo, 82, was held for political or financial gain.Mr Uduaghan said her captors had probably decided to let her go because they were under pressure."The army and police have been on their trail and a lot of raids have been done. I think because of the heat they dropped her off on the highway," he told the BBC's Focus on Africa programme.Dozens of people were reportedly detained on Thursday during the search for Mrs Okonjo.Earlier this week finance ministry spokesman Paul Nwabuikwu said that Mrs Okonjo-Iweala had been threatened recently.He confirmed Mrs Okonjo's release, but gave no further details, AP news agency reports.Driver arrestedIt is not clear if a ransom was paid.The governor said it was not government policy to pay ransoms but families sometimes negotiate with the kidnappers privately."For this one we also insisted that we would not pay any ransom [and] as a government we would not negotiate with anybody," he said.Mrs Okonjo, who was released at about 10:30 local time (09:30 GMT), was currently sleeping and would be questioned about her ordeal later, Mr Uduaghan said.The young man who had dropped off Mrs Okonjo has been arrested and is being questioned, he said.BBC Nigeria correspondent Will Ross says kidnappings are so common in the country that they rarely make front page news.Many businessmen, footballers and academics have been kidnapped for ransom in the south in recent months.In the past, only foreign oil workers used to be targeted, our reporter says.Mrs Okonjo-Iweala, 58, is an internationally respected economist who has led a high-profile campaign to clean up corruption in Nigeria, particularly in a controversial fuel subsidy programme.She has delayed the payment to fuel importers, seeking better verification of claims for subsidies.Mrs Okonjo is a medical doctor and retired professor of sociology at the University of Nigeria in Nsukka.Her husband, Obi Chukwuka Okonjo Agbogidi, a retired professor of economics, is the traditional ruler of the Ogwashi-Uku kingdom in Delta State.Mrs Okonjo-Iweala was one of the leading candidates to take over the World Bank this spring. She was previously the institution's managing-director but lost out to the Korean-American, Jim Yong Kim.