Lagos - Nigeria's anti-graft agency said on Tuesday it was detaining and questioning ex-president Goodluck Jonathan's spokesperson as part of a wider probe into corruption.
A source at the agency said the investigation touched on alleged misuse of funds allocated for fighting Boko Haram, Nigeria's notorious jihadist group.
"We have arrested Reuben Abati and he is undergoing interrogation in connection with some issues bordering on graft," Economic and Financial Crimes Commission spokesperson Wilson Uwujaren told AFP.
Uwujaren said Abati was arrested on Monday and was still in custody.
Another EFCC official, who did not want to be named, said Abati "is being questioned in relation to (an) ongoing investigation into the misappropriation of funds earmarked for arms to fight Boko Haram".
Abati was a popular columnist and chairperson of the editorial board of the independent Guardian newspaper before his appointment as Jonathan's spokesperson.
The journalist is accused of receiving money from former national security adviser Sambo Dasuki, currently facing a slew of charges over alleged bogus arms deals in which money meant for military procurements to fight Boko Haram was diverted for political purposes.
The funds were allegedly used to finance Jonathan's re-election in the 2015 vote which he lost to incumbent President Muhammadu Buhari.
Another EFCC source told AFP the former federal capital territory (Abuja) minister, Bala Mohammed, was also in custody.
"The FCT minister was arrested yesterday and he is being grilled in connection with some funds running into millions of dollars he allegedly misappropriated while in office," he said.
He said Mohammed, a former senator, was also being detained over "questionable" land allocations in the nation's capital.
"He is answering questions for allocating choicy plots of land to his sons and cronies," he added.
Buhari has launched a wide-ranging campaign against corruption targeting key members of the previous regime.
Former ministers, prominent party chiefs, media owners and relations of the former president are facing charges.
The focus on officials from the former ruling party has led some critics to claim Buhari is using the anti-graft crackdown as a way to silence political foes.
In a statement in Lagos on Tuesday, PRNigeria - a media consultancy group - quoted Jonathan as saying it was impossible for his former spy master, Dasuki, to have stolen $2.2bn as being alleged by the EFCC.
"I don't believe somebody can just steal $2.2bn," the former president said after his lecture at the famous Oxford Union in London.
"We bought warships, we bought aircraft, we bought lots of weapons for the army and so on and so forth and you are still saying 2.2 billion, so where did we get the money to buy all those things?," he asked.
Jonathan said corruption allegations under his watch were exaggerated.
"Yes, there were some issues. Yes, there are still corruption issues but some of it were overblown. I'd say exaggerated and they give a very bad impression about our nation. You cannot say the national security adviser stole $2.2bn. It is just not possible," he added.