MILANO- Italian tax police arrested 13 people, including executives of the Italian units of Germany's Siemens and France's Alstom, in an investigation into alleged bribery relating to Milan subway contracts, prosecutors said on Tuesday.

Among those arrested were officials at ATM, the Milan municipal transport company, and executives at Italian IT service provider Engineering Informatica and three other contracting companies, Milan prosecutors said in a statement.

The companies themselves were also being investigated, under an Italian law that holds firms liable for crimes by their managers, the prosecutors said, adding that the tax police were searching homes and offices in 15 cities.

An Alstom spokesperson told Reuters in an email the company had been informed about the investigation and was cooperating.

Siemens said the offices of Siemens Mobility in Milan had been searched and the company was cooperating with the authorities. It said it had zero tolerance for corruption or any violation of competition rules.

ATM said the allegations related to two of its officials. It denied the company itself had any role in the alleged activities and said it was cooperating with the investigation.

An Engineering Informatica spokeswoman said the firm had been notified. "We are looking into the circumstances of this investigation and do not have any further update at this time," she added.

Other firms did not immediately respond to requests for comment.

The prosecutors said those arrested were suspected of bribery, disrupting an auction, embezzlement, misuse of office and forgery in public acts.

The suspected bribes amounted to 125,000 euros paid or promised from October 2018 to July 2019 in relation to eight contract tenders worth 150 million euros.

(Reporting by Emilio Parodi; Editing by Gianluca Semeraro and Edmund Blair) ((emilio.parodi@thomsonreuters.com; +39 02 66129523;))