The document, seen by Reuters, said the investigation relates to the period between Jan. 1, 2015 and May 2, 2017 when the loss-making carrier asked to be put under special administration for the second time in less than a decade.

UniCredit boss Jean Pierre Mustier and Intesa Sanpaolo's Paolo Colombo sat on Alitalia's board during that time. The other people that were investigated by prosecutors in the town of Civitavecchia near Rome, include former Alitalia executives and other board members.

The document said prosecutors had looked into whether those investigated had committed bankruptcy fraud by approving financial statements that did not reflect the company's real state of health.

When wrapping up an investigation, prosecutors in Italy issue an official document that sums up the main elements of the probe and usually the next step is for prosecutors to request a trial.

UniCredit and Intesa Sanpaolo declined to comment. No comment was immediately available from lawyers for Mustier and Colombo.

The administrators running Alitalia have warned it faces possible liquidation if the government fails to engineer a rescue.

Analysts calculate that Italian taxpayers have spent more than 9 billion euros ($9.8 billion) propping up the airline, which has undergone two previous failed rescue attempts.

(Reporting by Domenico Lusi, writing by Valentina Za; Editing by Elaine Hardcastle)