Angolan authorities have detained the head of the Eduardo dos Santos Foundation (Fesa) that was founded by the retired leader.
Mr Ismael Diego was arrested on Friday over alleged misappropriation of $20 million and is being held at São Paulo prison in Luanda.
"Mr Ismael Diogo, José Eduardo dos Santos' close ally, was arrested following his failure to honour the Criminal Investigation Services summon," the spy agency confirmed.
"He even switched off his phone and moved to a different house.
The spy agency wants Mr Diogo to explain the alleged misappropriation of that amount from the Loaders National Counsel, under the management of former Transport minster, Mr Augusto Tomás, who was also arrested.
Fesa was founded in 1996 and has initiated and runs several projects in sectors including education, health, sports and culture. It also sponsors students to study abroad.
The source of Fesa's funds has been the subject of much speculation in Angola. Whereas some have argued that it is funded from public coffers, others, including its founders, maintain that it gets its resources from local and international donations.
Last week, Angolan authorities arrested and charged Mr Tomás over corruption.
Mr Tomás was also being confined at São Paulo prison. A Mr dos Santos's appointee, Mr Tomás was sacked by President João Lourenço last in June.
Mr Filomeno dos Santos, the son of the former president, was arrested last Monday for alleged involvement in criminal activities. He remains in custody in Luanda.
Filomeno was arrested alongside his business associate, Mr Jean Claude Bastos de Morais, for the suspect transfer of $500 million from the Angolan sovereign fund, which the former once headed.
President Lourenço has, since his ascent to power last year, embarked on an anti-corruption crusade that has seen several close associates of his predecessor implicated in wrongdoing.
He took over in August last year, following an election that marked the end of President dos Santos's 38-year reign.
President Lourenço sacked Filomeno as the head of the Strategic Sovereign Fund in January.
The president also sacked Isabel dos Santos, the former president's eldest daughter, as the chair of the board of state-owned oil firm Sonangol.
Ms Isabel dos Santos, who is Africa's richest woman, faces several graft investigations.