Official sources said the four-member team from Mumbai, which grilled Singh for nearly five hours yesterday, began questioning him at 10.30 am at Raj Rivas.
The entire proceedings are reportedly being videographed but Raj Nivas sources have remained tight-lipped on it.
Singh landed in a controversy after Bihar-based Congress leader Amlendu Pandey mentioned his name during questioning.
Singh admitted to having recommended expeditious issue of the passport when he was a Rajya Sabha member in 1997, though he did not know Khan, on humanitarian grounds as he was told that the Pune-stud farm owner needed to travel abroad urgently to visit his ailing brother.
The Prime Minister's Office had last week given clearance to the ED to record Singh's statement on the issue.
Opposition parties have called for a bandh tomorrow seeking recall of Singh, who is also caught in a controversy over the charge that a trust in which his two sons were former members had been given in-principle clearance by the administration to set up a medical college without following due procedures.
Singh has, however, found support in Chief Minister V Vaithilingam who has said "there is no necessity" for him to step down, saying he was facing only allegations and the ED would do "necessary statutory follow up."