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British police officers work on Westminster Bridge, adjacent to the Houses of Parliament in Westminster, central London on March 22, 2017, in the aftermath of a terror incident. Three people were killed and 20 injured in a 'terrorist' attack outside the British parliament Wednesday when a man mowed down pedestrians, then stabbed a police officer before being shot dead. The car struck pedestrians on Westminster Bridge, a popular spot with tourists because of its views of Big Ben, before crashing into the railings outside the heavily guarded parliament building in the heart of the British capital. / AFP PHOTO / Niklas HALLE'N (Photo credit should read NIKLAS HALLE'N/AFP/Getty Images)

British police officers work on Westminster Bridge, adjacent to the Houses of Parliament in Westminster, central London on March 22, 2017, in the aftermath of a terror incident.
Three people were killed and 20 injured in a 'terrorist' attack outside the British parliament Wednesday when a man mowed down pedestrians, then stabbed a police officer before being shot dead. The car struck pedestrians on Westminster Bridge, a popular spot with tourists because of its views of Big Ben, before crashing into the railings outside the heavily guarded parliament building in the heart of the British capital. / AFP PHOTO / Niklas HALLE'N        (Photo credit should read NIKLAS HALLE'N/AFP/Getty Images)

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British police officers work on Westminster Bridge, adjacent to the Houses of Parliament in Westminster, central London on March 22, 2017, in the aftermath of a terror incident. Three people were killed and 20 injured in a 'terrorist' attack outside the British parliament Wednesday when a man mowed down pedestrians, then stabbed a police officer before being shot dead. The car struck pedestrians on Westminster Bridge, a popular spot with tourists because of its views of Big Ben, before crashing into the railings outside the heavily guarded parliament building in the heart of the British capital. / AFP PHOTO / Niklas HALLE'N (Photo credit should read NIKLAS HALLE'N/AFP/Getty Images)

British police officers work on Westminster Bridge, adjacent to the Houses of Parliament in Westminster, central London on March 22, 2017, in the aftermath of a terror incident.
Three people were killed and 20 injured in a 'terrorist' attack outside the British parliament Wednesday when a man mowed down pedestrians, then stabbed a police officer before being shot dead. The car struck pedestrians on Westminster Bridge, a popular spot with tourists because of its views of Big Ben, before crashing into the railings outside the heavily guarded parliament building in the heart of the British capital. / AFP PHOTO / Niklas HALLE'N        (Photo credit should read NIKLAS HALLE'N/AFP/Getty Images)

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British police officers and emergency services work on Westminster Bridge, adjacent to the Houses of Parliament in Westminster, central London on March 22, 2017, in the aftermath of a terror incident. Three people were killed and 20 injured in a 'terrorist' attack outside the British parliament Wednesday when a man mowed down pedestrians, then stabbed a police officer before being shot dead. The car struck pedestrians on Westminster Bridge, a popular spot with tourists because of its views of Big Ben, before crashing into the railings outside the heavily guarded parliament building in the heart of the British capital. / AFP PHOTO / Niklas HALLE'N (Photo credit should read NIKLAS HALLE'N/AFP/Getty Images)

British police officers and emergency services work on Westminster Bridge, adjacent to the Houses of Parliament in Westminster, central London on March 22, 2017, in the aftermath of a terror incident.
Three people were killed and 20 injured in a 'terrorist' attack outside the British parliament Wednesday when a man mowed down pedestrians, then stabbed a police officer before being shot dead. The car struck pedestrians on Westminster Bridge, a popular spot with tourists because of its views of Big Ben, before crashing into the railings outside the heavily guarded parliament building in the heart of the British capital. / AFP PHOTO / Niklas HALLE'N        (Photo credit should read NIKLAS HALLE'N/AFP/Getty Images)

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British counter-terrorism police officers carry their weapons as they work on Westminster Bridge, adjacent to the Houses of Parliament in central London on March 22, 2017, during an emergency incident. British police shot a suspected attacker outside the Houses of Parliament in London on Wednesday after an officer was stabbed in what police said was a 'terrorist' incident. One woman has died and others have 'catastrophic' injuries following a suspected terror attack outside the British parliament, local media reported on Wednesday citing a junior doctor. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

British counter-terrorism police officers carry their weapons as they work  on Westminster Bridge, adjacent to the Houses of Parliament in central London on March 22, 2017, during an emergency incident.
British police shot a suspected attacker outside the Houses of Parliament in London on Wednesday after an officer was stabbed in what police said was a 'terrorist' incident. One woman has died and others have 'catastrophic' injuries following a suspected terror attack outside the British parliament, local media reported on Wednesday citing a junior doctor. / AFP PHOTO / Daniel LEAL-OLIVAS        (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

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A police Land Rover pulls a car transporter alongside the scene where a grey vehicle crashed into the railings of the Houses of Parliament in Westminster, central London on March 22, 2017, in the aftermath of a terror incident. At least three people were killed and 20 injured in a 'terrorist' attack in the heart of London Wednesday when a man mowed down pedestrians on a bridge, then stabbed a police officer outside parliament before being shot dead. Police guarding the iconic House of Commons building shot the man but several people were left with 'catastrophic' injuries on Westminster Bridge, a busy traffic junction popular with tourists with views of Big Ben. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)

A police Land Rover pulls a car transporter alongside the scene where a grey vehicle crashed into the railings of the Houses of Parliament in Westminster, central London on March 22, 2017, in the aftermath of a terror incident.
At least three people were killed and 20 injured in a 'terrorist' attack in the heart of London Wednesday when a man mowed down pedestrians on a bridge, then stabbed a police officer outside parliament before being shot dead. Police guarding the iconic House of Commons building shot the man but several people were left with 'catastrophic' injuries on Westminster Bridge, a busy traffic junction popular with tourists with views of Big Ben. / AFP PHOTO / Daniel LEAL-OLIVAS        (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)