Italian financial police searched the Milan offices of US bank JP Morgan on Thursday as part of an investigation into Monte dei Paschi di Siena's (BMPS) 2007 acquisition of the Spanish bank Antonveneta, the Corriere della Sera daily said.
A court in Siena is leading the probe, and police were looking for documents related to the case, the newspaper said, adding that the Milan branch was among several business offices that had already been searched in May 2012 as part of the same investigation.
The financial police and JP Morgan's Milan offices were not available for comment.
BMPS, which was founded in 1472 and is the world's oldest surviving bank, acquired Antonveneta from the Spanish bank Santander for nine billion euros ($11.75 billion), and the investigation has been looking into why the amount paid was so high.
The deal prompted BMPS to rebuild capital through a serious of dodgy derivatives trades which sparked a separate investigation into Italy's third-largest bank.
On Tuesday, Italian police were given orders to seize 1.8 billion euros destined for Japan's Nomura bank as part of that probe.