The Economic and Financial Crimes Commission (EFCC) has traced the diversion of the substantial part of the missing $20billion oil cash to the 2015 electioneering campaign of the Peoples Democratic Party (PDP).
The anti-graft agency discovered how the cash was transferred and wired into secret accounts for political purposes.
It was learnt that the EFCC has made "appreciable progress" in recovering a huge chunk of the cash. Former Central Bank of Nigeria (CBN) Governor Sanusi Lamido Sanusi (now Emir of Kano Muhammadu Sanusi II) raised the alarm that there was an estimated $20bn shortfall in oil revenues due to the treasury from the state oil company which might have been spent illegally.
The EFCC latest discovery is said to be part of the ongoing "comprehensive" probe of a former Minister of Petroleum Resources, Mrs. Diezani Alison-Madueke.
A source close to the investigator, said: "We have tracked a substantial part of the missing $20billion about which the former CBN Governor raised the alarm. "From our findings, the oil money was diverted. We will soon make the details available.
"As for those being invited for interrogation on campaign funds, we have been able to establish that the amounts in question were strictly government funds. "Whatever noise anyone is making is baseless. We are determined to recover these funds from all the beneficiaries. "Some of the suspects have refunded the cash credited to them and some still have outstanding funds to pay."
The highly-placed source said the EFCC was not targeting PDP leaders at all. He said if there is any evidence of government funds spent on the campaign of other political parties, we will "investigate too".
In 2014, Sanusi told members of the Senate Committee on Finance at the National Assembly in Abuja that $20billion was missing.
He said: "It is established that of the $67 billion crude shipped by the Nigeria National Petroleum Corporation (NNPC), between January 2012 and July 2013, $47 billion was remitted to the Federation Account." "It is now up to NNPC, given all the issues raised, to produce the proof that the $20billion unremitted either did not belong to the Federation or was legally and constitutionally spent."
Mrs Alison-Madueke denied that $20billion was missing. She said: "When the former CBN Governor came up with the allegation that about $49.8bn of crude oil sales proceeds were not remitted by NNPC, we swung into action with all the relevant agencies to reconcile the figures.
"When we reconciled the figures down to $10.8bn and some agencies were having a different figure of $12bn, we insisted that the reconciliation must continue because there was still an anomaly; we said we must get down to the bottom of the matter to find out what is really amiss.
"We appeared before the Senator Makarfi Commttee to defend ourselves over the fresh allegation of unremitted $20bn. We put our papers forward. The Makarfi Committee then went through all the evidence supplied by all the relevant agencies and found that there was no missing $20billion.
"Despite the fact that the Makarfi Committee declared that no money was missing, the opposition kept insisting that $20bn was missing and they were calling us names."
While receiving APC chieftains from Adamawa in April 2015, President Muhammadu Buhari promised to probe the whereabouts of the $20billion.
He said: "Going by the huge sum of money involved, there was no way an APC-led administration that has the fight against corrupt practices as a cardinal objective would sit by and watch some highly placed Nigerians loot the treasury."