MILAN (Reuters) - The long-standing head of Italian infrastructure group Atlantia ATL.MI is ready to step down as a probe into a deadly bridge collapse snowballs, prompting its top shareholder, the Benetton family, to pull support, sources said on Monday.

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Giovanni Castellucci, who masterminded a multi-billion merger with Spanish rival Abertis last year, will hand in his resignation at an extraordinary board meeting on Tuesday, two sources close to the matter said.

“His position is very difficult, I am not sure if he will find any support from investors,” one of the sources said.

The 60-year-old Castellucci, who has been Atlantia’s chief executive since 2006, was re-appointed with a new three-year mandate last May.

He did not immediately respond to an emailed request for comment by Reuters.

The Benetton family, which owns a 30.25% stake in Atlantia, signaled its backing for the manager was wavering after allegations of falsified safety reports at the group’s road operations emerged.

Italy’s tax police said on Friday they had found evidence that safety reports for some viaducts operated by Atlantia’s motorway unit had been falsified or information omitted, with the aim of misleading transport ministry inspectors.

One employee at Atlantia’s motorway unit Autostrade per l’Italia and two officials at maintenance arm SPEA were placed under house arrest as part of the probe, which follows the collapse of a bridge operated by Autostrade in the city of Genoa that killed 43 people.

Shares in Atlantia fell 7.8% on Monday, adding to an 8.4% drop on Friday.

At the weekend Benetton holding company Edizione issued an unusual, sharply worded statement saying it was dismayed by the revelations of the probe, which focused on several motorway viaducts other than the one that collapsed last year.

“In the light of the latest events, Edizione ... will without hesitation and immediately take all necessary actions ... to safeguard the credibility, reputation and good name of its shareholders and subsidiaries,” the holding company said.

A source close to the matter said the statement marked a “sharp change in the attitude of the Benetton family” toward the company and its management.

The Edizione board met on Monday but did not issue any statement.

Castellucci and other top managers at Atlantia are under investigation, together with top officials at Italy’s transport ministry, in a separate probe into the collapse of the Genoa bridge.

The Genoa disaster last year put the company under heavy political pressure, with ruling coalition party the 5-Star Movement demanding Autostrade be stripped of its motorway concession, which accounts for one third of its core profit.