"The net is widening and it's not clear how deep the rabbit hole goes. There will be a few people looking over their shoulders."
Nigeria has arrested the chairman of a local oil firm, a security official said on Tuesday, as part of a widening graft investigation in Africa's biggest petroleum producer that has also netted former oil minister Diezani Alison-Madueke.
Days after British police detained Alison-Madueke, one of Africa's most powerful women, the official said their counterparts in Nigeria's Economic and Financial Crimes Commission (EFCC) had arrested Atlantic Energy chairman Olajide Omokore on corruption and money laundering charges.
The arrests, which follow pledges by President Muhammadu Buhari - a former military dictator - to "clean up" Africa's biggest economy, have sent shockwaves through Nigeria's globe-trotting corporate and political elite.
"A lot of people have been shocked," one oil executive in Nigeria told Reuters. "The net is widening and it's not clear how deep the rabbit hole goes. There will be a few people looking over their shoulders."
The EFCC official said Omokore, listed by Forbes magazine in 2012 as one of "Ten Nigerian multi-millionaires you have never heard of", remained in custody.
"He is being interrogated," the official, who did not want to be identified, told Reuters. He gave no further details.
Omokore's lawyer and a spokesman for his company could not be reached for comment.
Alison-Madueke, whose house in Abuja was raided by Nigerian police on Friday night, was accused by former central bank governor Lamido Sanusi of presiding over a state oil sector that leaked tens of billions of dollars during her five-year tenure.
Sanusi was sacked by then-President Goodluck Jonathan after raising concerns that $20 billion in oil revenues had not been remitted to government coffers by the state oil firm, NNPC, between January 2012 and July 2013.
Sanusi also criticised Alison-Madueke for giving Atlantic - then an unknown start-up - a major contract with the NNPC shortly after becoming oil minister in 2010. She left office in May after Buhari's election victory over Jonathan two months earlier.
Alison-Madueke has denied any wrongdoing. Attempts to reach her in London have been unsuccessful.
The arrests have turned the spotlight once again on Nigeria's oil sector and the suspicions that oil theft, graft and mismanagement have created an ultra-rich elite in an economy typified by poverty, power cuts and shaky public services.
Atlantic Energy was one of two firms that signed so-called Strategic Alliance Agreements with NNPC’s upstream arm, NPDC, in May 2011 to manage oil blocks sold by oil major Royal Dutch Shell.
At the time, parliament criticized the two deals, which had a combined value of $6.6 billion, as having been subject to no bidding process.
Sanusi also questioned the logic behind them, saying they were simply transferring state assets into private hands. He identified the set-up as one of the three main methods through which he believed the NNPC was losing billions.
The Natural Resource Governance Institute, a U.S.-based watchdog, has also criticized the NPDC for holding onto substantial revenues despite having limited operational costs. It made an estimated $6.28 billion over a 19-month period in 2012-2013.
Since coming to power, Buhari has ordered an audit of NNPC, the central bank and other key state institutions, saying as much as $150 billion was stolen from the state over the previous decade.
Four months after being sworn in, Buhari has failed to nominate a cabinet - to the chagrin of the business and investor community - but has made clear he will preside personally over the petroleum sector to push through reforms.
A cabinet list sent to parliament on Tuesday included Emmanuel Ibe Kachikwu, a former Exxon-Mobil manager appointed to the helm of the NNPC in August after Buhari sacked the previous board in the first salvo of his reform drive.
Kachikwu's portfolio was not specified but oil industry sources say he is set to become state petroleum minister overseeing daily operations under Buhari.
It was not immediately clear who will run NNPC in his stead. Oil industry sources also said Buhari might also appoint a special advisor for oil.
Since taking office, Buhari has overseen the cancellation of controversial crude-for-product swaps and a costly refinery crude supply contract.
NNPC head Kachikwu announced an audit of the state company by PriceWaterhouseCoopers that is expected to be completed in December, sources familiar with the matter said.
The NNPC also said it wants to review upstream production contracts and split NNPC into smaller units to improve efficiency and end corruption.