The incident left three people, a man and two women, injured, according to the Metropolitan Police.
A man who had received life-threatening injuries was no longer in critical condition, officials said, adding, a woman who had less serious wounds was discharged from hospital.
Another woman, who had suffered lacerations from shards of glass after an officer discharged his weapon, was still being treated in hospital, Lucy D’Orsi, deputy assistant commissioner in the Metropolitan Police, said.
“The incident was quickly declared as a terrorist incident,” D’Orsi said, adding that the armed officers who shot Amman dead were part of a “proactive counter-terrorism surveillance operation.”
Meanwhile, Prime Minister Boris Johnson said in a statement, “An investigation is taking place at pace to establish the full facts of what happened,” promising his government would announce plans on Monday for making changes to the system for handling people convicted of terrorism offenses.
In November 2018, Amman pleaded guilty to possession of terrorist documents and disseminating terrorist publications, and the following month, he was sentenced to more than three years in prison.
Amman, 20, was released in January, but was deemed sufficiently high risk so he has been under special monitoring by police ever since.
“He was under surveillance, that is what allowed police to do their job so quickly. It could have been much worse than it was,” a Whitehall source said. “There had been concerns when he was in prison but there were no powers for any authority to keep him behind bars.”
According to witnesses, Amman had been wielding a machete at the time of incident and had silver canisters strapped to his chest, believed to be a suicide vest, which the police later declared, was a hoax.
Chris Wells, another witness, who was in a shop in Streatham with his daughter, said he heard three gunshots outside.
“People just came running in screaming and upset, shouting about a gun. We ran to the back of the shop and were locked in,” he told Reuters. “We tried to leave to get away and I saw a man in a hoodie with a gun, which I now know was a plain clothes officer,” he said. “And another officer shouted at us to get back inside because there was a bomb threat.”
Eyewitness Gulled Bulhan, a 19-year-old student from Streatham, told Skynews that “I was crossing the road when I saw a man with a machete and silver canisters on his chest being chased by what I assume was an undercover police officer – as they were in civilian clothing.”
“The man was then shot. I think I heard three gunshots but I can’t quite remember.”
The police have uncovered downloaded material on Amman’s computers and telephone about making explosives and carrying out terrorist attacks.
Messages showed that he had discussed with his family, friends and girlfriend his extreme views and desire to carry out an attack, often focused on using a knife, police said.
In December 2017, Amman posted a picture of Daesh leader Abu Bakr al-Baghdadi, who was killed during a US raid in Syria in October, and told his brother in a message that the Takfiri group “was here to stay”.
He also described Yazidi women as slaves and said it was permissible to rape them, and in another message, he encouraged his girlfriend to behead her parents.
Police said he had shared an online al-Qaeda magazine with his family and while in a discussion about school with a sibling, he wrote he would “rather blow myself up”.
The last time such an incident took place in London was in November at London Bridge. On that occasion, police shot dead a man, Usman Khan, wearing a fake suicide vest, who had stabbed two people to death and wounded three more before being wrestled to the ground by bystanders.
Khan was also a Daesh sympathizer, who had been jailed for terrorism but released early.
Like Khan’s attack, the new one will prompt further political debate about sentencing for terrorists.
An immediate investigation into the shooting is also being undertaken by the Independent Office for Police Conduct. The watchdog said, “It is mandatory for us to conduct an independent investigation when the police fatally shoot a member of the public.”
London’s mayor Sadiq Khan said in a statement, “Terrorists seek to divide us and to destroy our way of life – here in London we will never let them succeed.”