Further tests will be carried out concerning the deaths of two soldiers - including one who lived in London - during SAS selection training in the Brecon Beacons.
Edward John Maher and Lance Corporal Craig John Roberts collapsed on one of the hottest days of the year while climbing south Wales' highest mountain.
An inquest at Brecon Law Courts in Powys, mid Wales, gave the medical deaths of the pair as "unascertained".
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The Metropolitan Police said the building material was found in the force's former training facilities.
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