Nigerian anti-corruption officers have raided the offices of the agency which regulates the sale of fuel, as people return to work after the end of a week-long strike over the cost of petrol.
The Economic and Financial Crimes Commission says items were confiscated as an investigation into alleged fraud in the importation of fuel begins.
Nigeria is Africa's biggest producer of oil but imports refined fuel.
The strike was called after a subsidy on the cost of fuel was removed.
The government says the subsidy costs $8bn (£5.2bn) annually and that the money would be better spent on infrastructure and social services and said the biggest beneficiaries of the fuel subsidy were the owners of fuel-importing companies - among the richest people in the country.
But it backed down after the strike and agreed to reintroduce a partial subsidy.
Although unions responded by calling off their strike, some civil society groups have urged people to continue protesting.
EFCC spokesman Wilson Uwujaren told the BBC the Petroleum Products Pricing Regulatory Agency (PPPRA) offices in Lagos and the capital, Abuja, had been raided.
Report in various Nigerian newspapers say that the state oil firm - Nigerian National Petroleum Corporation - will also be subject to the EFCC investigation.
On Monday, the government approved the reduction of the pump price of petrol to 97 naira (about $0.60) per litre, restoring part of the fuel subsidy.
The price of petrol had risen from 65 naira ($0.40; £0.26) to 145 naira when the subsidy was removed without warning.
The protests led to the deaths of several people and in the commercial capital, Lagos, on sixth day of the strike police fired live bullets into the air and tear gas to disperse demonstrators.
The Nigeria Labour Congress and the Trade Union Congress said they had agreed to call off the strike in order to save lives - and said they welcomed the government's promise to explore corruption in the oil sector.
The BBC's Tomi Oladipo in Lagos says people were heeding the unions' call to return to work on Tuesday morning and the streets of Nigeria's biggest city were clogged with the usual heavy traffic.
However, some civil society groups have been angered by the decision to end the strike as petrol prices have not returned to the original pre-new year price.
"We cannot stop this kind of protest when the original aim of the protest has not been achieved," Mallam Shehu Sani, head of the Kaduna-based Civil Rights Congress, told the BBC's Network Africa programme.
"I can tell you from the perception of the public, they are disappointed that the unions have accepted 97 naira," he said.
BBC correspondents say some of that anger is felt in the cities of Kaduna in the north and Lagos in the south, even though people are returning to their jobs.
But the BBC's Chris Ewokor in Abuja said there was a sense of relief in the capital during the morning commute that the strike was over.
Oil accounts for some 80% of Nigeria's state revenues but, after years of corruption and mismanagement, it has hardly any capacity to refine crude oil into fuel.
The subsidy has meant fuel was much cheaper in Nigeria than in neighbouring countries, so large amounts ended up being smuggled abroad.