PARIS — The former chief executive of LafargeHolcim, the world’s largest cement maker, and five other top officials are being formally investigated as part of a government inquiry into whether the company helped finance the Islamic State militant group and other armed factions while operating a factory in Syria.

The expanding investigation underscores the difficulty of doing business in war-torn regions, a challenge faced by companies around the world, especially those in the energy and industrial sectors.

Lafarge, a sprawling French multinational with operations in 80 countries, has said that its employees were motivated to keep the Syrian plant running by a “can-do” approach. But critics have accused the company of trying to maintain profitability and a presence there, despite the growing danger to its employees.

Investigators questioned Eric Olsen, the former chief executive, on Thursday, a day after he was taken into police custody. They are trying to determine whether he and other company managers placed the lives of workers at the cement factory, in northern Syria, in danger during a civil war.