Two University of Maryland students were burned Monday afternoon after an explosion sparked a small fire in the chemistry lab, authorities said.

The incident occurred about 12:30 p.m. in a first-floor laboratory in Wing 3 of the university’s Chemistry Building. Fire officials said an experiment involving nitrogen and sulphur acid was underway when the a violent chemical reaction occurred, though precisely what caused the blast remains unclear. Combining nitrogen and and sulphur acid normally does not cause “such a violent reaction,” Prince George’s County fire officials said.

Two students, both women, in the immediate area of the explosion suffered from first- and second-degree chemicals burns and superficial lacerations, authorities said. Thy were transported to an area hospital in good condition. Two others in the lab had to be decontaminated but were not taken to the hospital, authorities said.

Firefighters had to extinguish a small blaze in the lab’s ventilation hood, and hazmat personnel went in shortly thereafter to investigate what might have caused the explosion. As of about 3 p.m., the Chemistry Building was still closed.

This post has been updated.