The former chief executive of Franco-Swiss cement group LafargeHolcim Ltd, Eric Olsen, was placed under formal investigation on Thursday as part of an inquiry into the group's activities in Syria, a source with the prosecutor's office told Reuters.

Olsen resigned as LafargeHolcim's CEO in April 2017, after the company admitted it had paid armed groups to keep a cement plant operating in Syria.

Three other individuals were put under formal investigation last week and two others – one of which being Lafarge's previous CEO Bruno Lafont – were still being questioned on Thursday evening, the source said.