More than 20 children were killed and dozens injured when a fire broke out in a school in Niger’s second-largest city on Monday morning, according to authorities in Maradi.

Three school classrooms made with straw were consumed by the fire, killing children aged between three and eight, local media reported.

The cause of the fire and where it started was not immediately known and an investigation was launched, the Regional Direction of National Education confirmed.

Straw huts are often used to make room for students in overcrowded schools in Niger.

In April, a fire fueled by high winds swept through an elementary school on the outskirts of Niger’s capital, Niamey, killing 20 children.

Since then, teachers and parents said that the deaths highlight the dangers of temporary classrooms set up outside.