Nigeria's anti-corruption agency has failed to tackle corrupt politicians effectively, a report by the US-based group Human Rights Watch says.
While the Economic and Financial Crimes Commission (EFCC) had prosecuted 30 politicians, only four were convicted and none are now in prison, it said.
But the report said not all the EFCC's failures since 2002 were its own fault.
It said that Nigeria's political system continued "to reward rather than punish corruption," it said.
"When ruling party chieftain Olabode George emerged from prison in 2011 after serving a two-and-a-half year sentence following a landmark EFCC prosecution, he was treated to a rapturous welcome by members of Nigeria's political elite including former President [Olusegun] Obasanjo and then-Defense Minister, Ademola Adetokunbo," it said.
Human Rights Watch said it showed that "proven criminality" was "no bar to the highest echelons of politics in Nigeria".
Despite its criticism, the report, Corruption on Trial?, said the EFCC was the most promising agency to confront corruption in the country and had recovered some $11bn (£6.7bn) through its efforts in the eight years since it was formed.
"The EFCC has had significant success in tackling private sector corruption, and from a human rights standpoint public sector corruption is what is key because endemic government corruption in Nigeria has undermined the basic rights of millions of Nigerians," HRW researcher Eric Guttschuss told the BBC's Focus on Africa programme.
But he mentioned the removal of EFCC head Nuhu Ribadu in the middle of his term in 2007, two weeks after the body arrested a wealthy former governor who was close to then-President Umaru Yar'Adua.
"The message was clear, if you go after power you pay a price," Mr Guttschuss said.
In response to the report, EFCC spokesman Femi Babafemi told the BBC some of the recommendations would "strengthen the anti-graft work of the commission".
But he said HRW's analysis that the EFCC had secured only four convictions of high-profile political figures was too narrow.
"The report limits the facts used to the cases and former governors and former ministers alone, whereas we have quite a lot of other high-profile cases."
He said that many cases were still before the courts, including those top government officials, former ministers and former governors.
"And I can also confirm to you that in a matter of weeks, more cases of this nature will be going to the courts," he said.
HRW said Nigeria's courts were sometimes "an obstacle to accountability", with most of the EFCC's big cases stalled for years without the trials commencing.