An explosion at a Beijing Jiaotong University science lab killed three students on Wednesday, the city’s fire department reported.

The blast occurred in the morning while students were conducting an experiment on a treatment for landfill leachate — water contaminated from passing through waste — in the university’s environmental engineering lab. Photos show fire and heavy smoke outside the building where the blast occurred. The fire was “under control” within an hour.

Local authorities are still investigating the cause of the accident. (Image: @希qqqqwq on Weibo)