The unknown substance was handed over to security, cops two days later.
The Sun
They then discovered a tube taking the gas to the patient's face mask had been deliberately blocked with an unknown substance.
The Sun
A vial found, a used syringe, an unknown substance.
Times, Sunday Times
A search of the car he was driving found glassine envelopes containing an unknown substance.
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A torus a thousand light-years in diameter, constructed of an unknown substance, rotating at a large fraction of lightspeed.
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