Siena, January 29 - Prosecutors in Siena have opened a probe against the former head and other top managers of troubled lender Monte dei Paschi di Siena (MPS). According to judicial sources, prosecutors are probing former president Giuseppe Mussari as well as other executives for ''fraud with respect to the shareholders of Montepaschi,'' as the Siena-based bank - the world's oldest - is called. The new probe is based on a series of claims presented by some of the bank's shareholders to Italy's stock market watchdog, Consob, and to the Bank of Italy, from 2008 to 2011. The claims focus on Montepaschi's takeover of smaller lender Antonveneta, which, according to investor claims, was carried out with insufficient diligence, without the ''necessary authorizations required by banking laws'' and for a sum - nine billion euros - which allegedly overvalued it and caused a strain on MPS's finances. The Siena-based lender is also embroiled in a crisis over lossmaking derivatives trades which could cost it some 720 million euros. The European Commission recently gave provisional backing to a 3.9-billion-euro Italian government bond issue for the bank ''to preserve the stability of the Italian financial system''. Meanwhile, authorities confirmed Monday that the bank faces at least two tax audits.