While Nuhu Ribadu was at the EFCC, eight ex-state governors were charged
The former head of Nigeria's anti-corruption unit has had threats made against his life, US-based Human Rights Watch (HRW) has said.
Nuhu Ribadu was removed from his post at the head of the Economic and Financial Crimes Commission (EFCC) last year and sent on "study leave".
HRW says he has since been subjected to an "escalating campaign of harassment".
Mr Ribadu told HRW that his car had been shot at in September and he has received threatening phone calls.
HRW called on the government to protect Mr Ribadu.
A spokesman for the police said the force had not received any complaints or reports of threats from Mr Ribadu or the police bodyguards assigned to him.
Mr Ribadu was moved from his post to attend a course a few weeks after the EFCC arrested a wealthy former governor from Nigeria's oil producing region, James Ibori, who was instrumental in getting the current president elected.
Despite promises from President Umaru Yar'Adua that there was "zero tolerance" from his government to corruption, the sleaze watchdog has been "seriously undermined", said Georgette Gagnon, HRW's Africa director.
During Mr Ribadu's tenure at the head of the EFCC the anti-corruption agency brought charges against eight former state governors, a former inspector general of police and several high profile businessmen, as well as a number of advance-fee conmen, known as "419ers".
But critics said his management of the EFCC's prosecutions were selective, and that the agency was being used by former President Olusegun Obasanjo to punish his enemies.
Allies of Mr Obasanjo had escaped prosecution, it was claimed.
The BBC's Andrew Walker in the capital, Abuja, says Mr Ribadu's sacking may have been part of an attempt to remove the influence of the former president in government.
HRW said it is concerned at the direction the EFCC has taken since the new head Farida Waziri was appointed.
The agency's top investigators were returned to the police force, and have been reassigned to states where the men they investigated retain great influence and power, HRW said.
But a spokesman for the EFCC said there was nothing unusual about officers being reassigned, and it had little effect on their investigations.
"It's old news, and not an issue. In the police you don't stay in one place forever, and no one individual officer is so important they cannot be replaced," the EFCC's Femi Babafemi said.