EFCC is edging towards former first lady, Patience Jonathan with the prosecution of one of her husband's aides, Dudafa.
Jonathan's special advisor on household and domestic matters Waripamo Dudafa is accused with two others and four companies of laundering millions of dollars in stolen public funds.
Former first lady Patience Jonathan claimed last week in a letter to Nigeria's anti-graft agency that she is the rightful owner of the over $15 million in multiple bank accounts, according to Nigerian news agency Sahara Reporters.
She told the Economic and Financial Crimes Commission (EFCC) that she had been withdrawing the money from the accounts to pay for her overseas health expenses.
The accounts were frozen in July, and she is arguing that the EFCC is trying to "harass or harangue her and short-change her of her personal funds in breach of her fundamental human rights".
The seizure of funds is "illegal, unlawful, wrongful and prejudicial" according an application she submitted to the federal high court in Lagos on Thursday demanding the release of the money.
Two other men pleaded not guilty along with Dudafa, while the representatives of the four companies pleaded guilty to charges of money laundering and forgery. The trial is adjourned until September 27.
Anti-corruption campaigners said the Nigerian government should investigate the ex first lady, who has attracted attention from anti-graft agencies and activists in the past for her vast personal wealth.
Debo Adeniran, head of the Coalition Against Corrupt Leaders organisation, said she had "guts" to claim the money is hers.
"I only hope the present government will have the political will to pursue a trial to the end," Adeniran said.
President Muhammadu Buhari, who defeated Jonathan in elections last year, has launched a wide-ranging campaign against corruption targeting key members of the previous regime.
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 target and silence political foes.
Former national security advisor Sambo Dasuki is currently facing a slew of charges over allegedly bogus arms deals in which money meant for military procurements to fight Boko Haram was diverted for political purposes.
Meanwhile, Jonathan's cousin Robert Azibaola is also standing trial for allegedly stealing millions in public funds.
But despite the arrest of his key confidants, Jonathan has yet to be charged with any offence.