3. Lal Bahadur Shastri
Death: January 11, 1966
He was the first Indian Prime Minister to die while in office. Lal Bahadur Shahtri and the then Pakistan President Ayub Khan met in Tashkent for a summit which was organized by Soviet prime minister Alexei Kosygin. The Tashkent declaration was signed between India and between Pakistan at this summit, in which the two armies returned to their respective positions after the 1965 war, with India handing over the vital Haji Pir to Pakistan.
That night, Shastriji died of multiple heart attacks. His wife alleged that he was poisoned. Suspiciously, his body was blue after he died which points finger at him being poisoned and not dying of heart attack. The butler who served Shahstri was arrested but was later freed due to lack of evidence.
In 2009, a journalist Anuj Dhar requested the government to carry out a probe into the mysterious death of Shastri. A report was released after some days which said that the prime minister's office has some documents related to Shastri's death but they cannot be released in public as it would harm India's foreign relations and create national unrest.
The Russian butler's arrest triggered many questions and suspicions but these queries still remain unanswered.