A fire in a nine-story apartment building in Vietnam’s capital killed at least 56 people, including at least four children, and injured at least 37, authorities said Wednesday

The fire started just before midnight Tuesday in a building housing about 150 residents. Firefighters contained the blaze, but the building’s location at end of a narrow alley made rescue operations difficult. Dozens of people remained trapped in the building until after dawn, state media reports said.

Of the 56 people confirmed dead, the police have identified 39 victims, the state-owned Viet Nam News said Wednesday evening, citing Hanoi police. State-owned national television channel VTV said four children were among those killed. Initial reports about the death toll were unclear since the injured and dead were taken to different hospitals across the city.

Many of the dozens being treated at hospitals suffered from smoke inhalation and injuries sustained during desperate attempts to escape the building.

Authorities are investigating the cause of the fire, which started around midnight in the parking area of the building, which had no emergency exit. The police have detained the building’s owner as part of their investigation.

The building was a ‘tube house’ — a narrow, elongated house that is several stories high — and its residents included families and students. Images of the building showed its walls blackened by soot, with wires around it mangled by the heat.

Vietnamese Prime Minister Phạm Minh Chinh visited the building and extended his condolences to families of the victims.

“We must take this as a lesson” to improve fire prevention and firefighting, he said, adding that “regulations must be taken seriously so we can avoid a tragedy like this.”

Last year, a blaze at a karaoke parlor in southern Vietnam’s Binh Duong province killed 32 people.