The Virginia Department of Labor and Industry and local authorities are investigating to find the cause of an explosion at a middle school construction site that caused serious injuries to eight workers.

The blast at the Crittenden Middle School happened while construction crews were repairing a roof that was damaged in recent storms. A 20-foot by 50-foot wall collapsed.

While it’s not known exactly what caused the explosion, a local TV station reports that the workers were using propane to melt tar being used to make roof repairs.

Two workers remain hospitalized in a burn unit, their conditions not released to the public. Four others are still hospitalized, one in critical, two in serious and one in stable condition. The two other workers were treated and released.

The school was closed for one day. After inspectors determined that most of the school was still safe, students returned, but several classrooms near the collapsed wall won’t be able to be used for some time.

Tags: burns, construction site, explosion, serious injuries