They are believed to have radio signals that reach only short distances, unlike more-sophisticated cellphone-activated detonators, the officials said.

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Security forces seized rocket-propelled grenades, explosives and detonators, as well as night vision binoculars, monitoring cameras, computers, fake passports and ID cards.

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Mr. Lenz added that as unexploded bombs grow older, they are becoming ever more unstable, and increasingly likely to explode, as the elements deteriorate their chemical detonators.

WSJ: German City to Evacuate as 2-Ton Bomb Is Defused

The injury happened after detonators - which are used as fog signals - were taken from a depot in Leigh-on-Sea and put on on a bonfire at a nearby park.

BBC: Essex

In recent times, personnel met at the bunker when flash flooding struck Inverness in 2002, also after a freighter carrying 84 tonnes of diesel and 3, 300 of zinc concentrate ran aground on the Summer Isles in 2003 and in 2010 after a container holding detonators exploded in Inverness.

BBC: Bunker mentality: The Cold War bunkers still in use