The EFCC boss said the agency is doing its best to arrest the fugitive.
While speaking on Channels TV on Monday, October 30, Magu said Maina's controversial reinstatement to service despite his fugitive status was 'devastating'.
He also rejected criticism of the anti-graft agency for not promptly apprehending Maina, even though he also expressed his dissatisfaction with the state of the investigation, promising to tackle "grey areas and loopholes".
He said, "When I came in, the matter was already in court. I was so busy pursuing other issues. You need to be here with us to see the volume of work we have.
"I really don't know what happened, but I know that he has been properly placed on the watch list and he cannot sneak into this country without the law enforcement agencies, you know, something is wrong somewhere and we need to look at that.
"I was not in the EFCC as at the time the investigation was conducted. I advise Maina to come and expose those who are involved because we will go after everybody. I am telling you, we will not spare anybody. Let him come out and expose them. If I'm involved in any way, I should also be exposed.
"He has people who are protecting him and that has made it difficult for us to get him arrested. You can imagine, he was promoted. It is really devastating, it is beyond my understanding."
He further appealed to Nigerians to help the agency in the manhunt for Maina and called for better cooperation in the war against corruption as it is Nigeria's "greatest enemy".
Maina was recalled and promoted after he had been dismissed by the Federal Civil Service Commission in 2013 following a recommendation by the Office of the Head of Service.
In 2015, he was charged alongside former Head of Service, Steve Oronsaye, Osarenkhoe Afe and Fredrick Hamilton Global Services Limited before a Federal High Court on a 24-count charge bordering on procurement fraud and obtaining by false pretence through the award of fraudulent biometric contracts.
After he was declared wanted over an alleged N2 billion pension fraud, he evaded capture from the EFCC and fled abroad.
Minister of Justice and Attorney General of the Federation, Abubakar Malami, Minister of Interior, Abdulrahman Dambazau, and Head of Service, Mrs Winifred Oyo-Ita, have been reported to be instrumental to the process of Maina's recall.
On Monday, October 23, after his reinstatement drew public condemnation, President Buhari ordered Maina's 'disengagement' and ordered a full investigation into the circumstances of his recall.
On Tuesday, October 24, the Senate and House of Representatives also set up separate ad-hoc committees to investigate the issue.