"If you can't make a bomb because family, friends or spies are watching or suspecting you, take a knife, molotov, sound bombs or a vehicle at night and attack the tourists (crusaders), police and soldiers of taghut or Western Embassies in every country you are in this planet", he wrote to her, the BBC said. Specialist officers established that a device strapped to the suspect's body was a hoax explosive.
Three victims were taken to local hospitals, with one man's condition "life-threatening" and a woman described as receiving non-life threatening injuries. "We will provide more details as soon as they are available".
The December attack was carried out by a man who had served time in prison for terrorism offenses. But now the incident has been fully contained and there are no additional attacks.
Police shouted for people to "get back" as they cleared the area of shoppers and bystanders.
London Mayor Sadiq Khan urged community resolve in the face of another attack.
In one, filmed from inside a shop across the road, a man can be seen lying on the street while at least two armed police officers point their guns from behind an unmarked vehicle with its blue lights flashing.
Reports indicate the shot man may have been wearing a vest of some kind. But I think the main thing for them to hear is people do these awful things, but not to be afraid.
Mr Bulhan said people began running into nearby stores after the incident. "Armed police and roads closed off".
Several unverified videos of the scene have been posted on Twitter.
"I heard two people got stabbed and apparently one man was shouting Allahu Akbar but people thought he was just a insane man, so he got away".
Images on social media show a person face down on the ground outside Boots, with a police officer holding a gun.
Chris Phillips, the former head of the National Counter Terrorism Security Office, told the BBC the incident "looks very similar to what we saw on London Bridge just a few months ago". British tabloids said the young man walked free from jail days before his rampage.