Delta state Gov. Emmanuel Uduaghan told the BBC the kidnappers likely let the 82-year-old Okonjo go because they were under pressure from the state.
The retired professior and mother of Finance Minister Ngozi Okonjo-Iweala needed some rest before she would be questioned about the ordeal, Uduaghan said.
"The army and police have been on [the kidnappers'] trail and a lot of raids have been done. I think because of the heat they dropped her off on the highway," the governor said.
The driver who freed Okonjo was arrested and was being questioned.
It is unclear whether Okonjo, who was taken from her home in southern Nigeria Sunday, was abducted for financial or political gain, officials said.