Prosecutors in Rome on Monday grilled outgoing mayor Ignazio Marino over allegations that he paid for thousands of euros of private meals with a municipal credit card.
The investigation was opened following charges levelled by the rightwing Fratelli d'Italia and the grassroots Five-Star Movement parties.
Marino formally resigned as mayor on 12 October after losing the support of his centre-left Democratic Party over the expenses scandal.
Marino said his resignation should not be taken as an admission of guilt, but said "political conditions" had made it impossible for him to carry on.
He had earlier offered to pay back 20,000 euros of expenses.
Supporters of Marino - considered by many to be honest but bumbling and unpopular - suspect a concerted smear campaign linked to his role in exposing a criminal network with links to the far right which had thrived under his conservative predecessor Gianni Alemanno.
Alemanno was among dozens of politicians and businessmen implicated in a major probe into the alleged mafia network, which is suspected of rigging tenders and of creaming off millions of euros.
The first criminal trial of those arrested in the probe opens in Rome on Tuesday and Marino plans to attend the initial trial hearings.
In the dock are five people including the former head of the municipal garbage collection service.
The city government will seek civil damages in the trial, Marino announced before his resignation.
In a video posted on Facebook and in a letter published on Rome City Council's website after he resigned, Marino said he feared the return of graft to the capital under future administrations.
A commissioner will be appointed to run Rome from 2 November until fresh mayoral elections are held next Spring, which the Five Star movement has been widely tipped to win.
