Retired and serving police officers have stated that the investigation in the Sunanda Pushkar murder case was deliberately derailed by an official of the Delhi police, who acted allegedly under the influence of the then ruling party leaders. According to these sources, the probe was "influenced from the start to make sure that the truth behind her death never came out". Evidence was either ignored or tampered with or removed because of such influences, and a wall of secrecy was introduced to prevent the facts from coming out.

These sources, who have direct knowledge of the matter, stated that a high ranking police officer, who is very close to a Congress party leader — a minister in the previous United Progressive Alliance (UPA) government — played a key role in derailing the Sunanda Pushkar murder investigation. They claim that he did so at the behest of a senior politician, who was in constant contact with him during the entire period of the probe. Despite this alleged collusion being a known fact in Delhi police circles, the new government has yet to probe the role of this "well-connected" police officer in the Sunanda case.

Sources in the police, who are familiar with the case, said that mid-rank police officials (DCP level), who were among the first to handle the initial investigation, had unanimously opined that prima facie it appeared to be a murder case. Their assertions were ignored by their superior, an officer of the Joint Commissioner of Police (JCP) rank, who was supervising the case at the time, and who still has a charmed existence, owing to high-level political contacts. "Unless this officer is quizzed, the full facts behind those responsible for the murder will continue to elude the investigation," a senior officer familiar with the derailing of the investigation warned.

"The role of this JCP rank officer, who was earlier associated with the case, needs to be probed. The opinion of all the other officers junior to him, who felt that it was a murder case, was ignored by him," an official said.

Sources said that there was no denying that it was a massively botched-up investigation, which needed a comprehensive probe "in view of reports of political influence and even illegal gratification figuring in the way the death was investigated". "The injuries on Sunanda were passed off as self-inflicted and no effort was made to determine the motive for the death," an officer said, adding that "in view of her deep and motherly attachment to her son, who needed her constant attention, it is impossible to believe that she could have killed herself and left him alone in the world voluntarily".

According to officials, the carelessness on the part of the Delhi police can be made out from the fact that a team of officers, who visited the crime scene in November 2014, a full 11 months after the death of Sunanda, found fresh evidence including broken glasses and fluid marks. No one, apart from the Delhi police officials responsible for the earlier slip-up, knows why this evidence was not collected earlier.

Later, a medical board constituted by the All India Institute of Medical Sciences (AIIMS) to examine the death, accused the police of not furnishing essential evidence and documents needed for experts to form an opinion. "Despite repeated reminders, the Delhi police failed to submit circumstantial evidences (sic) and statements about the reason for physical injuries on Pushkar's body," the board had stated.

"They took one year to lodge an FIR. The Police Headquarters too mishandled the situation by not keeping the media updated about the development in the case, leading to floating of conspiracy theories. Initially, the case was given to the Crime Branch, which wanted to register an FIR immediately, but then the case was taken from them the very next day, once this intention became known. The final medical report, on the basis of which the Delhi police has now filed their FIR, contains nothing new. If the FIR was to be filed on the basis of the medical reports, it could have been done long back," the official added, saying that "justice demands the officer guilty of botching up the investigation be brought to account".

The Delhi police is now going through the social media accounts of both Sunanda and Shashi Tharoor to find out the identity of a woman called "Katy", who is stated to be the reason behind the frequent quarrels between the couple. Another individual, whom the investigating agency is focusing on, is Sandeep Sharma, who is known for his liaison skills and is stated to be close to a serving MP from West Bengal. Sandeep Sharma, who according to police sources, shared a very cordial relation with Sunanda, had allegedly invested in the now defunct Kochi team of the Indian Premier League.

According to officials, the role of the doctors who were involved in this case need to be probed too. "A Dubai based paediatrician 'voluntarily' issued a certificate, adding to the confusion, by stating that Sunanda used to 'bruise' easily. Why did he do it? Who asked him? All these were points of investigation which the Delhi police should have completed long ago," said a retired officer of the rank of commissioner of police.

Curiously, the doctor in question, Anil Gupta, who practises at Cooper health clinic, Dubai, and claimed in his certificate that he was a long-time family friend of Sunanda, failed to reply to the repeated emails sent by this newspaper.

Commenting on the matter, BJP leader Subramanian Swamy said that he always thought that it was an open and shut case. "I never had any doubts. It is because of threat of my PIL all this (investigation) happened, otherwise they were doing nothing for 8-9 months. Now the police is active and those elements who wanted to suppress this investigation, are no more in power. Therefore, I think, we will get to the truth. Any person has a right to life, you cannot take it away like this and hush it up," he said.

On Saturday, Swamy said that "Mr Tharoor didn't kill her." He alleged, "But Mr Tharoor knows who killed her and facilitated her killing and told lies and covered up (for the murderer). So, he will also be sent to jail, but he won't get capital punishment."