Lal Bahadur Shastri, India's third prime minister, died in a dacha in Tashkent, shortly after signing a peace agreement with the Pakistani president, Ayub Khan, on 11 January 1966, about four months after the end of the second war between the two countries.
The cause of death was reported to be a heart attack, but conspiracy theories about the circumstances of the death began to circulate almost immediately. Details of these are can be found on multiple online sources, examples being the Wikipedia page and this BBC article.
In 1977, the then Indian government established the Raj Narain committee to investigate the circumstances of the death of Prime minister Lal Bahadur Shastri. The records of that committee have never been made public. In May 2018, the Central Information Commission of India directed the Indian Prime Minister’s Office and ministries of External Affairs and Home to make the records of the Raj Narain committee public. At the time of writing, this has not yet happened. Updates to this ruling may be published on the CIC website.
However, the Medical Report on the death of Lal Bahadur Shastri was released earlier this year. That shows that he did, in fact, die from a heart attack, and gives some detail on the treatment that he received.
An interpretation of the Medical Report has been published by Dr K.K. Aggarwal, President of the Heart Care Foundation of India & Past National President of the IMA. This suggests that the symptoms were consistent with those for a heart attack, and that the treatment was in line with the standards of the day.
A number of intelligence agencies have online repositories of records published in accordance with their national Freedom of Information laws (sadly, neither the FSB, nor its predecessor the KGB are among them). I searched several of these, and was unable to locate any declassified records about Lal Bahadur Shastri's death. The CIA FOIA Library does have some records relating to his time as Prime Minister, and the search for a successor, which you may find of interest.
Until or unless evidence to the contrary is published, the evidence suggests that Prime Minister Lal Bahadur Shastri died from a heart attack. There is nothing in the published records to suggest that there was anything suspicious about the circumstances of the heart attack, or the treatment that he received.
On the basis of the evidence currently available in the public domain, the death appears to have been from natural causes, and just happened to occur while the Prime Minister was abroad on an official trip.