"I must say that I am sure that we are on the right track and the Special Investigation Team is doing a great job," Bassi said, adding that a number of reasons were responsible for the "slow" progress in the case.
Tharoor, who is Lok Sabha member from Thiruvanathapuram, was questioned for nearly five hours by the SIT at the Anti-Auto Theft Squad office inside the premises of south Delhi's Vasant Vihar police station on Saturday, a source said. He was subjected to three rounds of questioning around one year ago.
On Tharoor's questioning, Bassi said, "Whatever clarifications were required to be obtained from Shashi Tharoor, those perhaps have been obtained."
"If SIT thinks further clarification is required, it may summon Tharoor again," he said, adding that the police is trying to take the case to a logical conclusion as early as possible.
The latest round of questioning was in light of the findings of the AIIMS Medical Board's opinion on the FBI report on Sunanda's viscera and other crucial evidence, which were sent to their laboratory in US, a police source said.
He said the questions revolved around the source of drugs Alprax, which was found in Sunanda's stomach, and lodicaine, which is believed to have contributed to a poisoning leading to her death. However, Tharoor has so far maintained that there is no foul play in Sunanda's death.
Meanwhile, the SIT has written a letter to the AIIMS medical board, seeking clarification on a few specific points in their findings. The board is expected to respond by Friday, the source added.
Sunanda was found dead at a five-star hotel room in south Delhi on January 17, 2014, and around a year after that, a case of murder was registered with the police calling it a case of "unnatural death".
In February 2015, the police sent her viscera samples and other evidence gathered from the hotel room to an FBI laboratory in US, which sent its report to Delhi Police in November last year.
However, the report failed to clear the mystery surrounding Sunanda's death and it was forwarded to an AIIMS medical board for their opinion.
The board was headed by Dr Sudhir Gupta, the chief of forensic science department of the institute, who claimed that the FBI report "endorsed" the report of Sunanda's autopsy which was conducted at AIIMS.
Around three weeks ago, the police had once again questioned Tharoor's domestic help Narayan Singh and driver Bajrangi in light of the AIIMS medical board's findings.
Singh and Bajrangi, along with Tharoor's family friend Sanjay Dewan and three others, were questioned mult ple times and later all six were subjected to a polygraph test last year.
Police also questioned around a dozen chemists from Lodhi Road area in the last week of January while trying to ascertain the source of drugs which are believed to have caused the death of 51-year-old Sunanda.
The AIIMS medical board, which gave its opinion on the FBI report on Sunanda's viscera and other samples, had unanimously concluded that she died of poisoning, with the presence of Alprax in her stomach.
The board did not rule out the possibility of any injectable poison, in view of a particular mark on Sunanda's body, and said that the FBI report confirmed the presence of lidocaine in her body.
It ruled out death due to combination of illicit drugs and also said that the medical board is "highly afraid" that this case may have led to a complete "botched-up" scenario in the absence of the FBI report.
It also said SIT probing the case should thoroughly investigate if any person had access to injectable agents in view of a syringe mark spotted in Sunanda's body.
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