File photo of Congress MP Shashi Tharoor and his wife Sunanda Pushkar
Six persons, including three "prime witnesses" in the Sunanda Pushkar death case, were put through a lie detector test, Delhi Police Commissioner BS Bassi said amid indication that a number of other witnesses may be asked to undergo the examination.
"Our investigation is continuing and whatever is required to be done will be done. So far we have carried out polygraph tests on six persons. If there is any requirement we will conduct further tests," Mr Bassi said.
Last month, the Delhi Police had approached a city court for permission to carry out polygraph test on Narain Singh, who is domestic help of Sunanda Pushkar's husband and Congress MP Shashi Tharoor, driver Bajrangi and family friend Sanjay Dewan.
The court had later allowed the investigators to conduct the polygraph test on the three suspects with their consent.
Three others - SK Sharma, Vikas Ahlawat and Sunil Takru, had also undergone the polygraph test.
Asked whether investigators were planning to subject Mr Tharoor to a polygraph test, Mr Bassi said he would not like to "speculate".
"I would not like to speculate but as and when anything of this sort is required we will let you know. But unnecessary speculation is not advisable," he told reporters.
Mr Tharoor has been questioned thrice in this case. Sunanda Pushkar was found dead in a hotel suite on January 17 last year, a day after she was involved in a spat with Pakistani journalist Mehr Tarar on Twitter over the latter's alleged affair with Mr Tharoor.
Investigators said they are trying hard to make a breakthrough in the high-profile case and are waiting a report from Federal Bureau of Investigation (FBI) to know about the kind of poison found in Sunanda Pushkar's body.
Her viscera samples were sent to the FBI lab in Washington in February after an AIIMS panel concluded that the poison cannot be detected in Indian labs.
Mr Bassi said the report of the polygraph test on the witnesses is awaited.
Police had accused Mr Singh, Bajrangi and Mr Dewan of concealing "material facts" like a power cut in room no. 345 at Leela Hotel on January 17, 2014 around 7 PM where Sunanda Pushkar was found dead around 8 PM.
In January, police had claimed that Sunanda Pushkar, 51, was poisoned, and registered a murder case against unknown persons.