How could a terror attack happen in Parliament? Westminster estate is one of Britain’s most guarded buildings

‘The Houses of Parliament are among the most heavily-protected buildings in Britain.
All visitors must pass through heavy security and airport-style scanners before they are allowed in.
So how could an assailant carrying a weapon get inside to attack a policeman?
It appears the breach happened at Carriage Gates, the main access point for cars and other vehicles to the Parliamentary estate.
These huge iron gates are directly opposite Parliament Square and just round the corner from Big Ben, where hundreds of tourists mill about throughout the day.’

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