Delhi Police commissioner BS Bassi said on Friday that it's certain the death of Sunanda Pushkar, wife of Congress MP Shashi Tharoor, was not natural.

"As far as evidences are concerned until now, Sunanda Pushkar's death was unnatural. I can say this with certainty," Bassi told reporters on Friday.

This announcement was made after the Delhi police received the AIIMS medical board's 'advice' on an FBI lab report on viscera samples from Sunanda Pushkar.

The report, prepared by top doctors, is based on the findings of the FBI, which, last year, was sent viscera samples (Pushkar's organs like the large intestine).

The FBI report virtually rules out the theory of 'polonium poisoning' having caused her death. However, after the Delhi police failed to draw any conclusion on the basis of their findings, the report was handed over to a medical board for examination before proceeding further in the high-profile case.

Bassi said the report is being studied by a special team of the Delhi Police set up to investigate Ms Pushkar's death.

The US agency came into picture after an AIIMS medical board identified that she died because of poisoning. The board did not mention any specific substance.

Sunanda was found dead inside her suite at a five-star hotel here in January 2014, a day after she was involved in a spat with Pakistani journalist Mehr Tarar on Twitter over the latter's alleged affair with Tharoor, the Congress MP from Thiruvananthapuram.

Investigators have so far conducted polygraph tests on six persons, all prime witnesses in the case, including the Tharoors' domestic help Narayan Singh, driver Bajrangi and Sanjay Dewan, a close friend of the couple.

The police has registered a case of murder and questioned Tharoor more than once; it has named no suspect.