"Someone inside 10 Janpath provided crucial information to the mole," says Pradhan in his just released book, "My Years with Rajiv and Sonia" (Hay House India), without identifying the "mole".
"I know for sure that Sonia Gandhi, who was away in Amethi virtually throughout the 1991 Lok Sabha election campaign, feels the same way," the 311-page book says.
Pradhan, who later became the Arunachal Pradesh governor, says although many suspects were arrested and some convicted for the former prime minister's 1991 killing, "I think it is ordained that the truth shall not come out".
Gandhi was the opposition leader when he was blown up at an election rally ground near Chennai by a woman suicide bomber of the now vanquished Liberation Tigers of Tamil Eelam (LTTE) May 21, 1991.
The LTTE denied involvement, but Indian investigators claimed the Tamil Tigers assassinated Gandhi to avenge the earlier deployment of the Indian Army in Sri Lanka's northeast.
Pradhan, who joined the Rajiv Gandhi team later, said that "on the basis of hindsight, one must admit that in the entire security planning in Delhi, the possibility of the LTTE making an attempt on the life of RG (Rajiv Gandhi) had been overlooked".
He says he once saw some Sri Lankans seated outside the 10 Janpath office of Vincent George, a long-time aide to the Gandhis.
"Obviously they had a contact at 10 Janpath who could arrange secret meetings with RG that not many knew of. I certainly did not."
According to Pradhan, the one person who could have saved Gandhi was then Tamil Nadu governor Bhishma Narain Singh. Tamil Nadu was then under President's Rule.
"Wasn't it the governor's responsibility to go into the whole security matter thoroughly? I have my doubts whether he applied his mind to the complexities involved."
Pradhan says that he specifically telephoned the governor then to review the security arrangements for Gandhi, who was leading the Congress election campaign.
Once Gandhi was killed, Pradhan says, he told the governor to resign if he had any self-respect and take the responsibility for the Tamil Nadu Police's failure to protect the former prime minister.
"I used very harsh language while talking to him (governor).
"What I said to Bhishma Narain Singh that day could be interpreted as my giving vent to anger, sorrow and mental agony, but the fact remains that despite several warnings, the governor did not apply his mind or use his powers to ensure adequate security."
"He was the only one who could have stopped Rajiv Gandhi from visiting Sriperumbudur (the assassination spot) that night even after RG had landed at the Madras (Chennai) airport."
Pradhan says that as a former home secretary, he was aware of the threats to Rajiv Gandhi's life.
"RG was the prime target of several organisations, including those of Sikh militants, the Sri Lanka-based LTTE and even the USA's Central Intelligence Agency (according to some sources).
"The Americans did not exactly appreciate RG's pan-Asian role and were wary of his returning to power with a fresh mandate."