London, May 27. Scotland Yard today was understood to have continued the drive started last week to round-up both Britons and aliens suspected of possible "Fifth Column" activity.
The police were said to be concentrating on persons never openly identified with any programme or movement" friendly to Germany but suspected of possibly advocating dangerous opinions in the event of an invasion of Britain.
Detectives searched 400 warehouses and buildings along the Thames River for arms and British uniforms which might have been hidden by German agents. River police boarded every ship entering the Thames. They detained for questioning some of the crews—United Press.
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