Anti-corruption officers in Nigeria have raided the offices of the PPPRA, the agency which regulates the sale of fuel in the West African nation.The Economic and Financial Crimes Commission says items were confiscated as an investigation into alleged fraud in the importation of fuel begins.The move comes as people return to work after the cost of petrol was cut following a week-long strike.Nigeria is Africa's biggest producer of oil but imports refined fuel.The strike, which lasted six days, was called after petrol prices doubled when President Goodluck Jonathan removed a fuel subsidy on 1 January, which the government says costs $8bn (£5.2bn) annually.It said 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.EFCC spokesman Wilson Uwujaren told the BBC the Petroleum Products Pricing Regulatory Agency (PPPRA) offices in Lagos and the capital, Abuja, were 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.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.