BELGRADE, Serbia (AP) — A house fire in a northern Serbian city early Friday killed six people, including four children, police said.

The fire erupted around 3 a.m. in Novi Sad, some 90 kilometers (55 miles) north of Belgrade, the capital, said Interior Minister Ivica Dacic.

An emergency doctor told state television channel RTS that all the victims were already dead when the medical team reached the scene.

Dacic said in a statement that initial findings suggest the cause could have been a charging electric scooter.

The children were between 2 and 7 years old, said Dacic. Police are working to establish the victims' identity but they assume they are all members of a family registered at the address, he added.

“This fire is a terrible tragedy and the whole of Serbia is mourning the tragic death of this family,” said Dacic in a statement. He said firefighters responded professionally and were at the scene swiftly after the emergency call.

The Novi Sad authorities declared three days of mourning, starting on Saturday.

Serbian media published photos from the scene showing charred items in a small, partly finished brick house in a poor neighborhood, including a baby stroller and a washing machine.