American tourist took picture with slain London officer Keith Palmer minutes before attack

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An armed police officer guards inside a police cordon outside the Houses of Parliament in central London on March 22, 2017 during an emergency incident.

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Keith Palmer, 48.

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A police officer is visibly upset as she's led away by a colleague at the scene outside the Palace of Westminster, London on March 22, 2017.

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Armed police officers stand guard as forensics officers work around a grey vehicle that crashed into the railings of the Houses of Parliament in central London on March 22, 2017, during an emergency incident.

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A member of the public is treated by emergency services near Westminster Bridge and the Houses of Parliament on March 22, 2017 in London, England.

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An armed police officer gets out of a car inside a police cordon outside the Houses of Parliament in central London on March 22, 2017 during an emergency incident.

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Injured people are assisted after an incident on Westminster Bridge in London, March 22, 2017.

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Armed police respond outside Parliament during an incident on Westminster Bridge in London on March 22, 2017.

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Injured person being treated at the scene in London, England on March 22, 2017.

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Emergency service workers arrive on the scene close to the Houses of Parliament in London on March 22, 2017.

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A bloodied hand emerges as a person is treated at the scene outside the Houses of Parliament London on March 22, 2017.

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Conservative MP Tobias Ellwood, pictured with a bloodied face, stands amongst the emergency services at the scene outside the Palace of Westminster, London on March 22, 2017.

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Armed police run the scene of the attack outside Parliament in London on March 22, 2017.

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Injured people are assisted after an incident on Westminster Bridge in London on March 22, 2017.

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Parliament Square and on Westminster Bridge went into total lockdown after a man plowed a car into a number of people on Westminster bridge pavement, crashed into the railings outside parliament and stabbed a police officer before the assailant was shot on March 22, 2017.

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Injured people are assisted after an incident on Westminster Bridge in London, March 22, 2017.

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Roads are closed off by police after sounds similar to gunfire have been heard close to the Houses of Parliament, London on March 22, 2017. The UK House of Commons sitting has been suspended as witnesses report sounds like gunfire outside.

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A member of the public is treated by emergency services near Westminster Bridge and the Houses of Parliament on March 22, 2017 in London, England. Scotland Yard report they have been called to an incident on Westminster Bridge where several people have been injured by a car.

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Staci Martin took a picture with slain London police constable Keith Palmer shortly before the Wednesday's attack at Parliament.

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Khalid Masood was shot and killed by the police after he mowed down multiple people on the Westminster Bridge then crashed into a House of Parliament and stabbed an officer to death on March 22, 2017.

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A man lies injured after a shooting incident on Westminster Bridge in London, March 22, 2017.

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Paramedics treat an inured person after an incident on Westminster Bridge in London on March 22, 2017.

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A member of the public is treated by emergency services near Westminster Bridge and the Houses of Parliament on March 22, 2017 in London, England.

Metropolitan Police

While police believe Khalid Masood acted alone in the deadly attacks, there may have been others who encouraged the UK born man to carry out the terror.

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A member of the public is treated by emergency services near Westminster Bridge and the Houses of Parliament on March 22, 2017 in London, England.

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A woman kneels down as she assists an injured person after a man hit a group of pedestrians on Westminster Bridge in London on March 22, 2017.

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Armed police officers enter the Houses of Parliament in London on March 22, 2017 after the House of Commons sitting was suspended as witnesses reported sounds like gunfire outside.

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A medic is seen near Westminster Bridge and the Houses of Parliament on March 22, 2017 in London, England.

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An armed police officer stands on Parliament Square outside of the Houses of Parliament in London on March 22, 2017 after the House of Commons sitting was suspended as witnesses reported sounds like gunfire outside.

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A woman lies in a pool of her own blood after she was mowed down by a car on Westminster Bridge during a two-pronged terror attack on March 22, 2017.

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Armed police officers enter the Houses of Parliament in London on March 22, 2017 after the House of Commons sitting was suspended following the attack.

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MP Tobias Ellwood (c) tried to save the life of Police Constable Keith Palmer, who was stabbed to death in the parliament attack by the assailant. Ellwood is pictured giving mouth-to-mouth resuscitation to the victim as he waited for medical staff and air ambulance to arrive at the scene.

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An armed police officer assists a woman after an incident on Westminster Bridge in London on March 22, 2017.