Undercover cops had seen the would-be killer buying items for a fake suicide belt two days before he attacked the man and woman.
As he was on licence, having been released from jail just 10 days earlier, he could have been arrested on suspicion of committing further offences and immediately sent back to prison.
Scotland Yard and MI5 did call an emergency meeting to discuss arresting Amman – dubbed one of the most dangerous terrorists in Britain. But the Probation Service opted not to recall him to jail for fear of blowing the round- the-clock surveillance operation on him, which aimed to unmask other terrorist activities.
Amman, 20, who was born in Coventry to a Sri Lankan family, was being tailed by armed police when he snatched an 8in knife from a shop in Streatham, south London, in February last year.
Dashing into the street, he yelled “Allahu Akbar” as he lashed out at passers-by. Amman stabbed two people before charging at two armed officers.
They shot him dead – 62 seconds after he grabbed the knife – the inquest into his death was told.
Yesterday, a jury concluded the two Scotland Yard marksmen lawfully killed Amman. But they also said the attacks might have been prevented had he been recalled to jail after buying four small bottles of Irn-Bru, kitchen foil and parcel tape from Poundland two days earlier. He used the items to make a hoax suicide belt and was wearing it when he was shot.
Amman had been living in a hostel in Streatham since his release from jail.
He was on licence after serving a 40-month sentence for collecting and distributing terrorist material.
The inquest was toldAmman was “one of the most dangerous individuals” ever investigated by police and MI5. A source said: “Amman was a ticking time bomb. The question was not if he would carry out an terror attack but when.”
During the inquest at the Royal Courts of Justice, a senior Scotland Yard officer denied a suggestion from a lawyer representing Amman’s family that the undercover operation had been a “massive failure”. He said the actions of the police on the day had prevented further tragedy.
Metropolitan Police Commissioner Dame Cressida Dick praised officers for “their professionalism, courage and decisiveness in the most challenging of circumstances – fast-moving, horrific and frightening”. She added: “The attack happened on a busy high street and their quick actions almost certainly saved lives.”
After the verdict, Coroner Mr Justice Hilliard also praised the bravery of the officers. They fired a total of six shots and hit Amman twice. Two bullets shattered windows and a woman was injured by flying glass.
In a statement read at the inquest, the male victim said he did not realise he had been stabbed until he saw blood pouring out of his side.
A couple rushed to his aid and plugged the wound with a sweater. He said: “I couldn’t believe it until I saw the blood hitting my hand and realised it was me. A man and a lady came over and tried to stop the bleeding. They cut off my top and saved my life. I will never forget them. The lady was crying, saying, ‘Please don’t go.'” He added: “This has affected me a lot.Thank God for the doctors, or I would be dead. My mind is not the same and I was very stressed. I am not sleeping. I cannot turn my head properly since the operation, I also cannot lift things.” Amman also stabbed a middle-aged woman who said: “I suddenly felt pain in my right shoulder towards my upper back. I took about four steps forward when I heard people say, ‘She’s been stabbed.'”
It emerged police put in a formal request for Amman’s release from HMP Belmarsh to be delayed. He had been involved in a protest at the top-security prison following the death of an inmate.
Carina Heckroodt, head of the London Extremism, Gangs and Organised Crime Unit in the National Probation Service told the inquest: “The police said Amman was a high threat and that an attack would be when, not if.”
She said she talked to Jenny Louis, governor of Belmarsh, about adding days to his prison sentence.
But Ms Louis said she had dealt with the incident and his time in jail could not be prolonged.
Deputy Assistant Commissioner Dean Haydon said after the verdict: “This case is a timely reminder that the terrorist threat is very real and has not gone away.”