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Traces of the World War One poison gas phosgene were detected in an incident at Sweden's security service HQ last Friday, official documents suggest.

Local media reports earlier said sensors on the building's roof detected phosgene, but authorities have not confirmed this.

The source materials for this need to be reactive, but that also usually makes them toxic, such as the commonly used phosgene.

Vinyl chloride is also dangerous when burned, as it produces toxic gasses like phosgene gas and hydrogen chloride.

Officials warned when they burned it that two concerning gases — hydrogen chloride and phosgene, which was used as a weapon in World War I — might be released in the process.