An armed police officer stands guard near Westminster Bridge and the Houses of Parliament on March 22, 2017 in London, England. (Photo by Jack Taylor/Getty Images)

A coded message posted on the 4chan online forum appeared to predict the terror attack at Westminster.

Four people were killed, including an unarmed police officer, and around 40 others were injured, after a knifeman brought terror to the heart of Westminster during a suspected terror attack on Wednesday. He was shot by armed police next to the Houses of Parliament.

The attack may have been flagged by a post on the forum, which contained references to its exact location. It also contained a photo, taken from the internet, of two guns.

It isn't known that the post was anything to do with the attacker, and it made no explicit reference to the date or nature of the incident. But many internet users are pointing out the chilling coincidence of the two events.

Expand Close Co-ordinates pointed to Westminster Bridge, which the attacker drove over / Facebook

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Whatsapp Co-ordinates pointed to Westminster Bridge, which the attacker drove over

The reference to the location was posted through a strange code. Alongside the picture, the user posted a series of dashes and dots – when translated into Morse code, they pointed to a URL, and at that page were the exact co-ordinates for the attack written in binary.

Those co-ordinates (51.500947, -0.124530) pointed to Westminster Bridge, which the attacker drove over. Specifically, they are located at the side where Parliament is located, which is where the shooting took place.

The post went published on the afternoon of 21 March, almost exactly 24 hours before the attack. It featured a note written on paint that included that date, making clear that it had been posted to 4chan's "Politically Incorrect" board.

Expand Close 4chan post also contained a photo, taken from the internet, of two guns / Facebook

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Whatsapp 4chan post also contained a photo, taken from the internet, of two guns

One user followed the code and posted "Big Ben?", just minutes after it went online.

Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Expand Previous Next Close A woman holds a placard up during a candlelit vigil at Trafalgar Square on March 23, 2017 in London, England. (Photo by Carl Court/Getty Images) Getty Images LONDON, ENGLAND - MARCH 23: A letter is left amongst candles during a candlelit vigil at Trafalgar Square on March 23, 2017 in London, England. Four People were killed in Westminster, London, yesterday in a terrorist attack by "lone wolf" killer Khalid Masood,52. Three of the victims have been named as PC Keith Palmer, US tourist Kurt Cochran from Utah and Mother of two Aysha Frade. (Photo by Carl Court/Getty Images) Getty Images LONDON, ENGLAND - MARCH 23: Members of the public light candles during a candlelit vigil at Trafalgar Square on March 23, 2017 in London, England. Four People were killed in Westminster, London, yesterday in a terrorist attack by "lone wolf" killer Khalid Masood,52. Three of the victims have been named as PC Keith Palmer, US tourist Kurt Cochran from Utah and Mother of two Aysha Frade. (Photo by Carl Court/Getty Images) Getty Images LONDON, ENGLAND - MARCH 23: People gather during a candlelit vigil at Trafalgar Square on March 23, 2017 in London, England. Four People were killed in Westminster, London, yesterday in a terrorist attack by "lone wolf" killer Khalid Masood,52. Three of the victims have been named as PC Keith Palmer, US tourist Kurt Cochran from Utah and Mother of two Aysha Frade. (Photo by Jack Taylor/Getty Images) Getty Images LONDON, ENGLAND - MARCH 23: Mayor of London Sadiq Khan looks on during a candlelit vigil at Trafalgar Square on March 23, 2017 in London, England. Four People were killed in Westminster, London, yesterday in a terrorist attack by "lone wolf" killer Khalid Masood,52. Three of the victims have been named as PC Keith Palmer, US tourist Kurt Cochran from Utah and Mother of two Aysha Frade. (Photo by Carl Court/Getty Images) Getty Images LONDON, ENGLAND - MARCH 23: A man looks on during a candlelit vigil at Trafalgar Square on March 23, 2017 in London, England. Four People were killed in Westminster, London, yesterday in a terrorist attack by "lone wolf" killer Khalid Masood,52. Three of the victims have been named as PC Keith Palmer, US tourist Kurt Cochran from Utah and Mother of two Aysha Frade. (Photo by Carl Court/Getty Images) Getty Images LONDON, ENGLAND - MARCH 23: People write messages of support on the ground during a candlelit vigil at Trafalgar Square on March 23, 2017 in London, England. Four People were killed in Westminster, London, yesterday in a terrorist attack by "lone wolf" killer Khalid Masood,52. Three of the victims have been named as PC Keith Palmer, US tourist Kurt Cochran from Utah and Mother of two Aysha Frade. (Photo by Jack Taylor/Getty Images) Getty Images A policeman points a gun at a man on the floor at the top of the frame as emergency services attend the scene outside the Palace of Westminster, London, after a policeman was stabbed and his apparent attacker shot by officers in a major security incident at the Houses of Parliament. PA PA 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. AFP/Getty Images AFP/Getty Images Handout photo issued by Metropolitan Police of PC Keith Palmer who was killed during the terrorist attack on the Houses of Parliament, London. PA PA British Police officers guard the entrance the The Mall, as the roads leading to Parliament Square in central London on March 22, 2017, remain cordonned off in the aftermath of a terror incident at the Houses of Parliament. AFP/Getty Images AFP/Getty Images A picture obtained from the Twitter account of James West, shows a car stopped on the sidewalk in front of the Palace of Westminster which houses the Houses of Parliament in central London on March 22, 2017 during an incident. James West/Getty AFP/Getty Images People leave after being evacuated from around the Houses of Parliament and Parliament Square in Westminster, central London on March 22, 2017 during an emergency 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 / DANIEL LEAL-OLIVASDANIEL LEAL-OLIVAS/AFP/Getty Images AFP/Getty Images A Metropolitan Police superintendent addresses the crowd from the pulpit at Westminster Abbey to some of the hundreds of people that were evacuated from Parliament during the terrorist attack. PA PA Police forensic officers close to the Palace of Westminster, London, after policeman has been stabbed and his apparent attacker shot by officers in a major security incident at the Houses of Parliament. PA PA LONDON, ENGLAND - MARCH 22: 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. A police officer has been stabbed near to the British Parliament and the alleged assailant shot by armed police. Scotland Yard report they have been called to an incident on Westminster Bridge where several people have been injured by a car. (Photo by Carl Court/Getty Images) Getty Images Armed officers attend to the scene outside Westminster Bridge and the Houses of Parliament on March 22, 2017 in London, England. A police officer has been stabbed near to the British Parliament and the alleged assailant shot by armed police. Scotland Yard report they have been called to an incident on Westminster Bridge where several people have been injured by a car. (Photo by Jack Taylor/Getty Images) Getty Images LONDON, ENGLAND - MARCH 22: Police and forensic officers at the scene of a terrorist attack in which a number of pedestrians were mowed down on Westminster Bridge on March 22, 2017 in London, England. Four people including a police officer and his attacker have been killed in two related incidents outside the Houses of Parliament and on Westminster Bridge in what Scotland Yard are treating as a terrorist incident. (Photo by Carl Court/Getty Images) Getty Images LONDON, ENGLAND - MARCH 22: An armed police officer stands guard outside the Ministry of Defence on March 22, 2017 in London, England. A police officer has been stabbed near to the British Parliament and the alleged assailant shot by armed police. Scotland Yard report they have been called to an incident on Westminster Bridge where several people have been injured by a car.(Photo by Jack Taylor/Getty Images) Getty Images LONDON, ENGLAND - MARCH 22: Police officers block access to the southern entrace of Westminster Bridge on March 22, 2017 in London, England. Four people including a police officer and his attacker have been killed in two related incidents outside the Houses of Parliament and on Westminster Bridge in what Scotland Yard are treating as a terrorist incident. (Photo by Carl Court/Getty Images) Getty Images Emergency services' vehicles are pictured on Westminster Bridge, adjacent to the Houses of Parliament in Westminster, central London on March 22, 2017, in the aftermath of a terror incident. A police officer was among three killed Wednesday in a suspected terrorist attack outside Britain's parliament by an assailant who was then shot dead by armed police, Britain's top anti-terror officer said. Mark Rowley said the two other victims died on Westminster Bridge in London after the attacker mowed down pedestrians in a car, leaving at least 20 people injured in total. / AFP PHOTO / Niklas HALLE'NNIKLAS HALLE'N/AFP/Getty Images AFP/Getty Images Ambulances are pictured outside St Thomas' Hospital in central London on March 22, 2017, in the aftermath of a terror incident at Parliament. A police officer was among three killed Wednesday in a suspected terrorist attack outside Britain's parliament by an assailant who was then shot dead by armed police, Britain's top anti-terror officer said. Mark Rowley said the two other victims died on Westminster Bridge in London after the attacker mowed down pedestrians in a car, leaving at least 20 people injured in total. / AFP PHOTO / Niklas HALLE'NNIKLAS HALLE'N/AFP/Getty Images AFP/Getty Images LONDON, ENGLAND - MARCH 22: A Police boat patrols the river Thames by St Thomas' hospital and Westminster Bridge on March 22, 2017 in London, England. A police officer has been stabbed near to the British Parliament and the alleged assailant shot by armed police. Scotland Yard report they have been called to an incident on Westminster Bridge where several people have been injured by a car. (Photo by Jack Taylor/Getty Images) Getty Images British Police officers stand betwen the pods of the London Eye, during the evacuation of passengers, in central London on March 22, 2017, in the aftermath of a terror incident at the Houses of Parliament. 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 / Joel FordJOEL FORD/AFP/Getty Images AFP/Getty Images The Elizabeth Tower at the Houses of Parliament, are pictured 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-OLIVASDANIEL LEAL-OLIVAS/AFP/Getty Images AFP/Getty Images Ambulances are pictured outside St Thomas' Hospital in central London on March 22, 2017, in the aftermath of a terror incident at Parliament. A police officer was among three killed Wednesday in a suspected terrorist attack outside Britain's parliament by an assailant who was then shot dead by armed police, Britain's top anti-terror officer said. Mark Rowley said the two other victims died on Westminster Bridge in London after the attacker mowed down pedestrians in a car, leaving at least 20 people injured in total. / AFP PHOTO / Niklas HALLE'NNIKLAS HALLE'N/AFP/Getty Images AFP/Getty Images LONDON, ENGLAND - MARCH 22: Ambulances, police vehicles and emergency services seen on Westminster Bridge on March 22, 2017 in London, England. A police officer was stabbed near to the British Parliament and the alleged assailant shot by armed police. Scotland Yard also reported an incident on Westminster Bridge where one woman has been killed and several people seriously injured by a car. (Photo by Carl Court/Getty Images) Getty Images LONDON, ENGLAND - MARCH 22: Ambulances, police vehicles and emergency services seen on Westminster Bridge on March 22, 2017 in London, England. A police officer was stabbed near to the British Parliament and the alleged assailant shot by armed police. Scotland Yard also reported an incident on Westminster Bridge where one woman has been killed and several people seriously injured by a car. (Photo by Carl Court/Getty Images) Getty Images LONDON, ENGLAND - MARCH 22: Police officers seen on Westminster Bridge on March 22, 2017 in London, England. A police officer was stabbed near to the British Parliament and the alleged assailant shot by armed police. Scotland Yard also reported an incident on Westminster Bridge where one woman has been killed and several people seriously injured by a car. (Photo by Carl Court/Getty Images) Getty Images Emergency personnel on Westminster Bridge, close to the Palace of Westminster, London, after policeman has been stabbed and his apparent attacker shot by officers in a major security incident at the Houses of Parliament. PRESS ASSOCIATION Photo. Picture date: Wednesday March 22, 2017. See PA story POLICE Westminster. Photo credit should read: Dominic Lipinski/PA Wire PA LONDON, ENGLAND - MARCH 22: Commander BJ Harrington of the Metropolitan Police makes a statement outside of New Scotland Yard on March 22, 2017 in London, England. A police officer has been stabbed near to the British Parliament and the alleged assailant shot by armed police. Scotland Yard report they have been called to an incident on Westminster Bridge where several people have been injured by a car. (Photo by Jack Taylor/Getty Images) Getty Images Police close to the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Victoria Jones/PA Wire PA 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. (Photo by Carl Court/Getty Images) Getty Images Police outside the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Victoria Jones/PA Wire PA An armed police officer stands guard near Westminster Bridge and the Houses of Parliament on March 22, 2017 in London, England. (Photo by Jack Taylor/Getty Images) Getty Images An Air Ambulance outside the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Victoria Jones/PA Wire PA An Air Ambulance outside the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Victoria Jones/PA Wire PA Police outside the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Victoria Jones/PA Wire PA Police close to the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Victoria Jones/PA Wire PA An Air Ambulance outside the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Victoria Jones/PA Wire PA Police outside the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. PRESS ASSOCIATION Photo. Picture date: Wednesday March 22, 2017. See PA story POLICE Westminster. Photo credit should read: Victoria Jones/PA Wire PA Police outside Westminster Abbey, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Victoria Jones/PA Wire PA MP's in House of Commons, London after a major security alert at the Palace of Westminster after a man apparently carrying a knife charged through the gates into the front yard of the parliamentary compound. PA Wire PA David Lidington, the Leader of the House of Commons speaking in the House of Commons, London after a major security alert at the Palace of Westminster after a man apparently carrying a knife charged through the gates into the front yard of the parliamentary compound. Photo credit should read: PA Wire PA Police outside the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Victoria Jones/PA Wire PA An Air Ambulance outside the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Victoria Jones/PA Wire PA Police outside the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Jonathan Brady/PA Wire PA Police outside the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Jonathan Brady/PA Wire PA Police close to the Palace of Westminster, London, after policeman has been stabbed and his apparent attacker shot by officers in a major security incident at the Houses of Parliament. Yui Mok/PA Wire PA Police outside the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Jonathan Brady/PA Wire PA Police close to the Palace of Westminster, London, after policeman has been stabbed and his apparent attacker shot by officers in a major security incident at the Houses of Parliament. Jonathan Brady/PA Wire PA Police close to the Palace of Westminster, London, after policeman has been stabbed and his apparent attacker shot by officers in a major security incident at the Houses of Parliament. Jonathan Brady/PA Wire PA Police outside the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Victoria Jones/PA Wire PA Police outside Westminster Abbey, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Victoria Jones/PA Wire PA Police outside the Palace of Westminster, London, after sounds similar to gunfire have been heard close to the Palace of Westminster. A man with a knife has been seen within the confines of the Palace, eyewitnesses said. Victoria Jones/PA Wire PA / Facebook

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Whatsapp A woman holds a placard up during a candlelit vigil at Trafalgar Square on March 23, 2017 in London, England. (Photo by Carl Court/Getty Images)

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