David Rossi's body was found on Wednesday lying beneath the open window of his third floor office in the bank's headquarters, a restored 14th century fortress. An autopsy was carried out on Thursday at the request of his family.
"The injuries are all compatible with the hypothesis of suicide," said the source, who has direct knowledge of the investigation into how Rossi died.
The unexplained death adds a further layer to a politically-sensitive scandal, the most complex seen in Italy since the accounting fraud at dairy firm Parmalat a decade ago.
Monte dei Paschi, Italy's third-largest bank, is at the centre of investigations into alleged corruption and fraud over the costly 2008 acquisition of Antonveneta bank and risky derivative trades.
Police have sealed off Rossi's office at the bank but it is unclear what, if any, impact his death will have on either the main investigation or on an insider trading probe opened on Tuesday. Prosecutors declined to comment on Friday.
The latest probe was triggered by leaks to the media about a Monte dei Paschi board meeting last week, at which it decided to seek damages from two former executives and investment banks Nomura and Deutsche Bank over derivatives losses, police said.
Police seized Rossi's cellphones and files from his computer as they seek to piece together his final days and hours, another judicial source said.
According to this source, prosecutors have found evidence of a long phone conversation that he had shortly before his death at around 8 p.m. on Wednesday.
Rossi had earlier sent a text to his wife saying he was leaving the office, the source said.
When Rossi failed to return home, his wife called his assistant, who checked his office. He found Rossi's jacket on a chair and the office window wide open, saw his body lying in the street and called the emergency services, the source said.
Rossi was one of the closest aides of Monte dei Paschi's former chairman Giuseppe Mussari, who is under investigation in both the Antonveneta case and the probe over derivative losses.
Rossi was not under investigation himself but his home and offices had been searched by police.
Rossi had seemed concerned and stressed, friends and people who had seen him in recent days said.
Another source with direct knowledge of the case said a crumpled scrap of paper was found in a waste basket in his office reading: "I have done a stupid thing".
Rossi had worked for more than a decade for Mussari, but he had been confirmed in his job even after his patron left the bank last year.
He was spokesman of Monte dei Paschi's foundation, the bank's biggest shareholder, from 2001 to 2006 and when Mussari, the foundation's chairman, moved to head the bank in 2006, Rossi went with him.
(Writing by Silvia Aloisi; Editing by Barry Moody and Mark Potter)
By Silvia Ognibene and Silvia Aloisi