The female police officer who was arrested over alleged media leaks about the "plebgate" affair was present at Downing Street during the time of the incident in September, the Metropolitan Police said.
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Armed police have carried out a raid apparently linked to Wednesday's deadly terror attack in Westminster.
Cheryl Grimmer vanished from inside a beach shower block in Wollongong, Australia, in 1970, when she was just three.
Pc Keith Palmer was on the front line and because of his actions, the Westminster attacker was unable to kill anyone else.