A fire that engulfed an Islamic school in Myanmar’s main city of Yangon killed 13 children who were trapped behind doors believed to be locked due to security concerns after sectarian violence ravaged parts of the country.

At least 13 boys suffocated after the fire broke out early Tuesday morning in a dormitory of the school, which also operates as an orphanage housing 75 children. The others managed to safely escape after police knocked down the door.

Police officer Thet Lwin told The Associated Press the fire was triggered by an overheated inverter and “not due to any criminal activity”.

“The fire, caused by the overheating of the transformer placed under the staircase, spread, trapping the boys sleeping in the attic. As a result, 13 twelve-year-old boys died of suffocation after inhaling smoke,” an officer from Yangon Region Fire Service said.

Some Muslims expressed concern since the incident followed a wave of communal violence in the Buddhist-majority country, reports Reuters.

Myanmar has been on edge after anti-Muslim violence erupted in the central city of Meikhtila, razing dozens of houses and killing dozens of people.