On Tuesday, the Delhi Police finally announced what many had long whispered about: businesswoman Sunanda Pushkar’s death last year wasn’t suicide. On Tuesday, the police registered a case of murder against unknown persons, as the final medical report prepared by the Department of Forensic Medicine and Toxicology of the All India Institute of Medical Sciences stated that the wife of Congress leader Shashi Tharoor had been poisoned to death.

Pushkar was found dead in a hotel room in New Delhi  on January 17 last year. The preliminary report from the autopsy carried out at AIIMS in January 2014 that her death was due to overdose of a prescription drug called Alprax, two strips of which was found in her room. The police also ruled out any connection to the dozen injury marks on her face and body to the case.

The final medical report by the three-member panel of the All India Institute of Medical Sciences in October contradicted the results of the preliminary investigation and claimed that traces of poison had been found in her body.

"The medical board gave us a report which says that the death was unnatural and due to poisoning," Delhi Police Commissioner B S Bassi told the media on Tuesday. "She died due to poisoning. Whether the poison was given orally or injected into her body is being investigated," he added.

The poison in question could be one of a number of substances found in her body that were undetectable in Indian labs, but can be detected in a foreign lab, The Hindu reported on Tuesday. These substances could include hallium, nerium oleander, snake venom, photolabile poison, heroin and polonium 210.

According to Times Now, the police believe that polonium 210 could be the cause of Pushkar’s death. The substance, which was discovered by Marie Curie in 1898, is highly radioactive. It also has a history of being discovered in controversial murder cases around the world.

This was the same substance found in the body of Yasser Arafat, who died in 2004. The concentration of the substance in the Palestinian leader's body was said to be “18 times the normal levels”.

Two years later, polonium 210 was also used to poison Alexander Litvinenko, a fugitive officer of the Russian secret service who specialised in tackling organised crimes. According to British prosecutors, his own friends slipped a “colourless, odourless substance into his tea”. Litvinenko died three weeks later.

How it works

Polonium 210 is hard to find naturally in deadly concentrations but it is manufactured for industrial use. It is used to eliminate static electricity in machines as well as in brushes for removing dust from photographic films. The substance is a big source of heat energy as well, according to Jefferson lab, a national laboratory in the US that conducts research on atoms and energy, “a single gram of polonium-210 creates 140 Watts of heat energy”.

According to the United States Nuclear Regulatory Commission, producing Polonium in a quantity large enough to harm someone would require it to be fabricated using sophisticated lab procedures.

“Once absorbed into the body it can be many times more toxic than cyanide,” it says in its backgrounder on polonium 210. "The alpha radiation can rapidly destroy major organs, DNA and the immune system."

Although it cannot pass through the skin, once ingested, “it damages tissues and organs", Philip Walker, professor of physics, University of Surrey, told  told BBC News.