Congress leader Shashi Tharoor has once again been questioned in connection with his wife Sunanda Pushkar’s death and the special investigation team probing the high-profile case may summon him again, Delhi Police Commissioner B S Bassi said on Sunday.

"I must say that I am sure that we are on the right track and the Special Investigation Team (SIT) 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.