While the cause of the devastating fire that destroyed Atlantic Sapphire’s Danish site last fall is still unknown, investigators have turned their attention to the water used to extinguish the fire, which has been found to contain high levels of a toxic substance.

The chemical perfluorooctanesulfonic acid (PFOS) was emitted from building materials containing plastic during the fire, Atlantic Sapphire CTO and co-founder Thue Holm told Danish news site Dagbladet Ringkobing-Skjern.

“PFOS is present in several forms of plastic products, including panels and pipes,” Holm told the site.