Nigeria's anti-graft agency has vowed to press on with corruption investigations in the northern state of Zamfara, despite a recent attack on a government office involved in a fraud probe.  The anti-corruption campaign has run into controversy amid allegations of bias.

The Economic and Financial Crimes Commission said the burning down of a government office in the northern state of Zamfara was to destroy evidence in ongoing corruption investigations.

The anti-graft agency last Monday filed charges against Governor Ahmad Sani and five other state officials for stealing more than $6 million in public funds.

Sani, who is seeking the main opposition All Nigeria Peoples Party's ticket to contest next year's presidential ballot, is immune from prosecution because of his office.

The EFFC spokesman, Osita Nwajah, says the circumstances of the incident points to arson perpetrated by officials desperate to cover up their misdeeds.

"We actually asked the commissioner of agriculture to come help us with our investigations and that he should bring all necessary documents relating to the matter we were enquiring into," he said.  "The man came and he turns up with absolutely nothing. And then a couple of days later, the office goes up in flames.  So, it is obvious that our suspicion will go in a certain direction. Yes, we do suspect sabotage and we suspect this was planned arson."

Nigeria, a leading oil exporter, is rated as one of the most corrupt countries in the world.  The EFFC says it has made several arrests and seizures in a high-profile anti-corruption campaign.

Critics say the federal agency, created by President Olusegun Obasanjo in 2003, has become vindictive and is being used by Mr. Obasanjo to settle political scores.

The EFFC has dismissed accusations of bias and has vowed to sanitize Nigerian politics ahead of elections in April.  Nwajah says the agency is determined to weed out corrupt politicians.

"We are already partnering with civil society groups to fight corruption and if ordinary Nigerians buy into this work that we are doing, we will almost have a 100 percent free and fair elections," he added.  "People who are going to be coming out and vie for elections, we would reasonably expect that they would have been scrutinized by the law enforcement agencies and the Nigerian people. And so, we definitely are going to have a better crop of leaders in 2007."

The EFFC says two-thirds of Nigeria's 36 state governors are suspected of corruption, but cannot be prosecuted due to constitutional immunity.

President Olusegun has publicly declared that he will thwart the ambition of those he described as corrupt politicians eager to succeed him.