Tharoor was quizzed for over four-and-a-half hours at the Anti Auto Theft Squad (AATS) office in South Delhi in two sessions.
Tharoor's domestic help Bajrangi and Narayan Singh, his common friend Sanjay Diwan, his PS Praveen Kumar and Rajat Mohan (cardiologist of Sir Ganga Ram Hospital) were questioned along with him. Besides, they were also separately quizzed by by a five-member SIT team here.
There were allegations that he had "misused" his office to ensure that Rs 70 crore, which was equivalent to 19 per cent equity in IPL Kochi franchise Rendezvous Sports, was paid to Sunanda, charges flatly denied by him.
Sources also said that a top senior police official, who is the member of SIT, left for the US to give the sample of Sunanda's viscera to Federal Bureau of Investigation (FBI).
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Police want to confirm whether the poison was radioactive isotopes, which cannot be detected in Indian labs, the sources said.
Delhi Police had earlier examined the former minister on January 19.
The SIT team, headed Deputy Commissioner of Police (DCP) Prem Nath, comprising Additional DCP P S Kushwaha and three senior officers asked Tharoor about his relations with Sunanda and others aspects to establish the theory.
DCP Mangesh Kashyap from the Economic Offences Wing (EOW), who was also part of the team, asked some questions to Tharoor about the now defunct IPL Kochi franchise.
Police Commissioner B S Bassi said, "We called six witnesses today in the second round of questioning into the case. The SIT confronted them and once again recorded their statements.