BRUSSELS, Belgium — Belgian police stopped a bus coming from France on Thursday after a passenger reported a conversation about a possible terror attack, officials said.

The bus, run by the company Flixbus and operating a service between the northern French city of Lille and Brussels, was halted in the town of Wetteren, next to Ghent, a spokeswoman for the East Flanders prosecutors’ office told AFP.

Police arrested three passengers and were to question all those on board to determine if the suspect conversation was substantiated, she said.

A spokesman for Justice Minister Paul Van Tigchelt said the passenger had overheard a suspected “terrorist conversation” and alerted police who stopped the bus and inspected it.

The federal prosecutors’ office said it was not treating the incident as a terrorist case for the moment.

Belgium’s official Coordination Unit for Threat Analysis said it was not raising the national terrorism alert level above its current status of “serious.”

Belgian police and security services are on heightened vigilance after the country faced recent Islamist attacks.

In 2016, jihadist suicide bombers killed 32 people with blasts in Brussels’ airport and metro system.

And in October last year, a Tunisian man shot dead two Swedish soccer fans in Brussels before being fatally shot by police.