Iswaran was arrested last July and subsequently released on bail.
It was later revealed that he was assisting the CPIB with investigations into a case it had uncovered, though the bureau did not elaborate on the nature of the investigation.
Prime Minister Lee Hsien Loong separately said that he had given CPIB the green light for the investigation, and that he had instructed Iswaran to take a leave of absence until the probe was completed.
Mr Ong the hotelier was also assisting in the investigations.
In response to media queries, the Attorney-General’s Chambers (AGC) said on Thursday that besides investigating Iswaran, CPIB also investigated the role of other persons including Mr Ong.
“The Attorney-General’s Chambers will take a decision in respect of the investigations against Mr Ong and others, after the case against Mr S Iswaran has been completed, including the presentation of evidence in court,” said a spokesperson.
Mr Lee told Parliament in August that Iswaran had been interdicted from duty with a reduced pay of S$8,500 a month until further notice.
The Prime Minister said then that since incidents involving ministers being probed for corruption are rare here, there was no rule or precedent on how to effect an interdiction on a political office holder. And as such, Mr Lee used the current civil service practice as a reference point.
The last time that a minister was instructed to take leave of absence amid a graft probe was almost 40 years ago.
In 1986, then National Development Minister Teh Cheang Wan was investigated by the CPIB for allegedly accepting two bribes totalling S$1 million in return for helping two property developers retain and acquire pieces of land for development.
Teh had been placed under leave of absence during the course of the investigations, but he died by suicide before he could be formally charged in court.
Before that in 1975, then-Minister of State for the Environment Wee Toon Boon was charged and subsequently convicted and jailed over four counts of corruption involving a sum over S$800,000.
Iswaran was first elected as an MP in 1997 and was appointed to the Cabinet in 2006.
He held ministerial positions in the Ministry of Communications and Information, Ministry of Home Affairs and Ministry of Education.
His latest portfolio was as Minister of Transport and Minister-in-charge of Trade Relations at the Ministry of Trade and Industry.
Any public servant convicted under Section 165 of the Penal Code can be sentenced to jail up to two years, be handed a fine, or both.
If found guilty of accepting or obtaining gratification under the Prevention of Corruption Act, Iswaran can be fined up to S$100,000, jailed for up to five years, or both.
If convicted with obstructing the course of justice he can be jailed for up to seven years, fined, or both.