* Giuseppe Mussari summoned as investigation widens
* Prosecutors target former bank management
* Rome prosecutors look at possible Vatican bank link
By Silvia Ognibene
SIENA, Italy, Feb 4 (Reuters) - Italian prosecutors called in the former chairman of Monte dei Paschi di Siena on Monday as they pursued their corruption investigation into an opaque series of loss-making derivatives trades at the bank.
Giuseppe Mussari, who stepped down as Monte Paschi chairman in April last year as the bank's problems mounted, was driven to the prosecutors' office in Siena by his lawyer and left without speaking to reporters.
One of his lawyers said his full legal team had not been available and he had sought a new appointment on Thursday with prosecutors, who are investigating a series of bribery and corruption allegations against former managers of the bank.
A former Dresdner Bank employee, Antonio Rizzo, who has been one of the key witnesses in the case, spoke to financial police on Monday and said he had repeated testimony he had already given to investigators.
Rizzo's testimony mentioned a group of former executives within Monte Paschi he said were known as the "5 percent gang" because of the commissions they were said to take on every deal.
Italy's third largest bank has been at the centre of a financial and political storm since last month when it revealed that it faced losses of some 720 million euros ($986 million from a series of derivatives and structured finance trades.
The losses, apparently linked to Monte Paschi's costly acquisition of smaller rival Antonveneta in 2007 have raised questions over the future of the Tuscan bank, which depends on a 3.9 billion euro state loan.
The case has also become a burning political issue, three weeks before Italians vote in a general election on Feb. 24-25 because of the tight links between Monte Paschi and regional politicians who dominate the shareholder foundation which controls the 540-year-old bank.
On Monday, Vatican spokesman Father Federico Lombardi denied a report in the Corriere della Sera daily which said that Monte Paschi executives had concealed secret payments related to the Antonveneta deal that were apparently routed through accounts at the Vatican bank IOR.
"The report is not reliable and the meetings it speaks of about Antonveneta never took place," Lombardi said.
Mussari, chairman of the bank until April last year, has been the target of stinging criticism over his stewardship and was forced to step down as head of the Italian banking association when the losses were revealed in January.
Monte Paschi executives are believed to have used the derivatives trades to massage accounts and conceal losses following the 9-billion-euro Antonveneta deal, which left the bank badly weakened just before the 2008 financial crisis.
The bank's new leaders and the Bank of Italy have accused the former management of concealing details of a complex derivatives operation linked to Italian government bond prices, which left Monte Paschi badly exposed when many European bond markets collapsed in 2011.
The Bank of Italy has faced increasing pressure over its supervision and Mario Draghi, who was governor of the bank before becoming president of the European Central Bank in November 2011, has also been drawn into the criticism.
Prosecutors are looking into allegations that the price tag for Antonveneta concealed massive bribes paid to facilitate the deal and that fraudulent accounting was used to hide the true nature of the derivatives contracts.
The case has been complicated by parallel investigations into other aspects, including a probe by prosecutors in the small southern town of Trani who are looking into allegations from a consumer group that the Bank of Italy was lax in its supervision of the sector.
Mussari pursued the Antonveneta deal to consolidate Monte Paschi among the front ranks of Italian banks. However even at the time eyebrows were raised when he paid around 9 billion euros in cash for a bank which Spain's Santander had acquired for just 6.6 billion euros only weeks earlier.
In addition to Mussari, prosecutors have targeted former managing director Antonio Vigni and the former head of the bank's finance department Gian Luca Baldassari as well as a string of other senior executives.