The Mexican government had offered to fly the couple out of the country, and days before he was scheduled to travel, Mr. Neruda was admitted to the Santa María clinic in the capital.

In 2011, Mr. Neruda’s driver at the time, Manuel Araya, publicly claimed that Mr. Neruda had not been in critical condition beforehand and that a day before his death Mr. Neruda, 69, told him that a doctor had given him an injection in the stomach that made him “burn inside.” His health quickly deteriorated. Mr. Araya contends that the poet was poisoned by doctors in the clinic, although there are no material witnesses to confirm the accusation.

Few of Mr. Neruda’s relatives and friends believe Mr. Araya’s version, nor does the Pablo Neruda Foundation, which manages his estate. But there are contradictory accounts regarding his health conditions and how advanced the cancer was, and it is not clear why Mr. Araya kept silent all these years.

Mr. Araya said that he had tried to tell Communist Party leaders at the time, but that no one would listen. Almost four decades later, he again approached the party to relay his suspicions about Mr. Neruda’s death. This time, the party filed a criminal lawsuit to have the courts establish the truth.

“Once there is a doubt, I believe it is extremely important to clear it up and use the technological means at our disposal to determine the poet’s cause of death,” said Judge Mario Carroza, who is in charge of the investigation. “We will not spare any possibility.”