* Bank spokesman found dead late on Wednesday
* Body found beneath open window of bank headquarters
* Prosecutors said investigating possible suicide
By Silvia Ognibene
SIENA, Italy, March 7 (Reuters) - Prosecutors began investigating the apparent suicide of the spokesman of Monte Paschi di Siena, the Italian bank subject to a corruption probe, and a judicial source said an autopsy would be done to dispel any doubt about how he died.
David Rossi, head of communications at the bank, was found dead late on Wednesday. His body was lying beneath the open window of his third floor office in the bank's headquarters, an eyewitness and the source told Reuters.
The source said police were not aware at this stage of any threat against him and another source with direct knowledge of the investigation said messages had been found that looked like suicide notes.
The bank itself paid tribute to Rossi, saying he had carried out his duties with "absolute ability and dedication in a particularly difficult period" but asked for a period of silence out of respect for his family.
"The death of David Rossi is a terrible tragedy," it said in a statement.
The unexplained death adds a further layer to a politically sensitive scandal, the most complex seen in Italy since the Parmalat accounting scandal a decade ago.
Monte dei Paschi, Italy's third-largest bank, is at the centre of an investigation into alleged corruption and fraud over the costly 2008 acquisition of Antonveneta bank and risky derivative trades.
Police have sealed off Rossi's office in a restored 14th century fortress in the Tuscan city of Siena 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.
The new 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.
Rossi was one of the closest aides of one of those executives, former chairman Giuseppe Mussari, who was already under investigation in the Antonveneta case.
Rossi was not under investigation himself but his home and offices had been searched by police.
Officials originally planned not to conduct an autopsy because the cause of death appeared obvious but changed their minds at the insistence of the family "to dispel any possible doubt", a judicial source with close knowledge of the case said.
"I HAVE DONE A STUPID THING"
A reserved, usually well-dressed man known by colleagues and journalists for his sharp and sometimes sarcastic manner, Rossi had seemed concerned and stressed in the past few days and could not understand why his house and office had been searched, one friend said.
"What have I done? Why have they come to me?," the friend quoted him as saying.
"Compared with his usual manner, he had been very worn down and quiet recently," said another person who had seen him in the past few days.
Officials said the manner of his death suggested suicide although there was no official confirmation pending the autopsy and the results of the investigation.
One 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".
Investigators also found some scribbled messages that appeared to be attempts by Rossi to write a farewell letter to his wife, the source told Reuters. They were seeking to identify the handwriting on the scrap of paper and the messages.
Rossi, whose father had recently died, 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.
Mussari, former head of the Italian banking association, has been questioned by magistrates over allegations of misleading regulators, market manipulation and false information in the prospectus of the Antonveneta deal.
The scandal triggered a political storm before last month's Italian parliamentary election because the bank has close ties to the centre-left Democratic Party, which won control of the lower house but failed to win the Senate.