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

WSJ: Bomb Suspect Was Put on Two Watch Lists

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

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