Being placed under investigation means that prosecutors believe they have "serious or consistent evidence" that could result in prosecution. It is a step towards a possible trial, though the investigation can still be dropped.

The men's lawyers made no further comment.

On Thursday, prosecutors also placed under formal investigation Lafont's successor Eric Olsen, former chief executive of Franco-Swiss cement group LafargeHolcim, the world's largest cement maker, which resulted from the 2015 takeover of Lafarge by Swiss Holcim.

Olsen quit as CEO in April after the company admitted it had paid armed groups to keep a factory operating in Syria. His lawyer has said Olsen will appeal against being put under investigation.

In June, French prosecutors launched an investigation into LafargeHolcim's operations and its possible "financing of a terrorist enterprise".

LafargeHolcim Chairman Beat Hass told French daily Le Figaro earlier this month the firm had made unacceptable errors in Syria and that it was cooperating with French investigators.

Two other former Lafarge executives and one LafargeHolcim executive still working for the group were also placed under formal investigation last week in the same case.

LafargeHolcim employs some 90,000 people in around 80 countries.

(Reporting by Emmanuel Jarry; Writing by Geert De Clercq; Editing by Alison Williams)