A married couple died of suffocation after an unknown suspect set a passenger bus on fire in downtown Moscow on early Thursday, the investigation committee said in a statement.

“According to witnesses an unidentified man, who later fled the scene, was sitting in the back of the bus and set on fire two plastic canisters with gasoline,” the statement said.

The fire broke out in the bus, that was about to leave for the city of Vladimir, as passengers were boarding the vehicle at around 6:00 a.m. Moscow time (2:00 GMT).

The married couple did not get out of the bus in time and died of carbon monoxide poisoning caused by the fire.