Here's an article on Marcinkus, where he is studied as a suspect in the possible murder of Pope John Paul I:
Paul Casimir Marcinkus (nickname: The Gorilla)
He was born January 15, 1922 in Cicero, Illinois, and ordained a priest in 1947. He received a Doctorate in canon law from Gregorian University, Rome and was posted to the English section of the Vatican Secretary of State's office, Rome.
After tours of duty in Canada and Bolivia, in 1959 he was appointed to the Secretary of State's department, Rome.
In 1964 he became the bodyguard to Pope Paul VI, acquiring the nickname, "The Gorillla." After accompanying the pope on several trips he had become also a personal translator and security advisor. Marcinkus became personal friends with the pope's secretary, Father Pasquale Macchi.
Several years later, he was made a bishop by Pope Paul VI and immediately made secretary of the Vatican Bank. Marcinkus had no previous banking experience! Pp. 102-105.
In 1973 he was investigated by the F.B.I. for direct involvement in money laundering of mafia money through the Vatican Bank.
On Sept. 28, 1978 Marcinkus found that he was to be replaced as head of the Vatican Bank by Msgr. Abbo on September 29.
Sept. 29. 6:45 a.m. Marcinkus in the courtyard near the Vatican Bank is told that Pope Jean Paul I had been found dead. Marcinkus's residence was not inside the Vatican; it's a 20 minute drive away. He was not known to be an early riser. His presence in the Vatican at that hour was never explained.
On Sept. 28, 1981, Pope John Paul II promoted Marcinkus to archbishop and pro-president of the Pontifical Commission for the State of Vatican City (a virtual governorship). He retained his post as head of the Vatican Bank.
Despite motive and access, Yallop does not place Marcinkus high on the suspect list.