Police in Nigeria are questioning an MP who headed an inquiry which found that a fuel subsidy scam had cost the country $6.8bn (£4bn).
Farouk Lawan is now being investigated over allegations that he tried to secure a $3m bribe from an oil firm to drop its name from the probe.
The legislator has denied asking for any bribes.
But on Tuesday local media said Mr Lawan had admitted receiving $500,000 and informed a parliamentary committee.
Meanwhile, MPs have voted to strip Mr Lawan of all his responsibilities in the House of Representatives.
Last month, Mr Lawan told the BBC he had received several death threats from powerful individuals after exposing the fuel scam.
The BBC's Will Ross in Lagos says many Nigerians are wondering if they will ever know whether he really took a bribe or whether this was a smear campaign to discredit the entire fuel subsidy report.
Mr Lawan chaired the House of Representative committee that produced the fuel scam report calling for a total overhaul of the oil ministry and for the prosecution of companies and some powerful individuals who had benefited from the swindle.
The fuel sector probe was set up in the wake of angry nationwide protests in January after the government tried to remove a fuel subsidy.
Nigeria is a major oil producer but has to import most of its fuel.
This week the oil tycoon, Femi Otedola, told media in Nigeria that Mr Lawan had demanded a bribe in order to have his company removed from a list of those involved in the scandal.
He said part of that money was handed over and a video recording of the transaction was given to police.
A spokesman for Nigeria's police said Mr Lawan was detained for questioning into these allegations on Thursday evening.
The initial fuel subsidy report said Mr Otedola's Zenon company owed more than $1m to the government.
Our reporter says in what remain unclear circumstances, legislators voted to remove the firm from the final report.