Delhi Police Commissioner B S Bassi said the high-profile case will be taken to its logical conclusion soon and all possible angles are being examined.
"One thing is clear that the death was not natural. It was unnatural as per our investigation till now and evidences collected so far. I can say that with certainty," Bassi told reporters here.
There was earlier speculation that Sunanda may have died as a result of poisoning through radioactive substances.
The FBI had sent its report to Delhi Police two months back. The report said the radiation levels in Sunanda's viscera samples were "within the standard safety norms" besides mentioning other details.
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As police could not firm up about cause of the death based on the FBI report, they requested a medical board of AIIMS to analyse the contents of the report.
51-year-old Sunanda was found dead at a suite in five-star hotel in South Delhi on the night of January 17, 2014, a day after her spat with Pakistani journalist Mehr Tarar on micro-blogging website Twitter over her alleged affair with Tharoor.
"I have no knowledge on the mention of any dangerous chemical in the FBI report. The FBI report had said none of the samples contained any radioactive material. I can confirm there's no radiaoactive material but certain other findings are there. We had given the entire report to the medical board which has examined and gave us a report.
was attended by the Special Investigation Team (SIT) probing the case. Special Commissioner of Delhi Police (Law and Order) Deepak Mishra briefed all about all related angle in the case.
"Have been told that Medical Board's advice in late Sunanda's case has been received," Bassi tweeted earlier.
Bassi added Mishra was reviewing the progress in the case.
The investigators have so far conducted polygraph test on six persons, all prime witnesses in the case, including Tharoors' domestic help Narayan Singh, driver Bajrangi and Sanjay Dewan, a close friend of the couple.
The FBI report had ruled out the theory of the variant of a radioactive element named Polonium having caused Sunanda's death, noting that radiation levels in her viscera sample were within the standard safety norms.
In January last year, police had claimed that Sunanda was poisoned, and registered a murder case against unknown persons.
Three others -- S K Sharma, Vikas Ahlawat and Sunil Takru, had also undergone the polygraph test.
The police team probing the case has so far questioned at least 15 people. Tharoor, his staff members and close friends were among those questioned by Delhi Police.