They increasingly believe it was an Islamist attack although they cannot rule out that the state of the terrorist’s mental health may also have played a part.

A source said: “Methodology wise, this attack is entirely jihadist – but we have an open mind as to what precisely was motivating him. If he built a much more destructive device then the detonation looks accidental. It looks lucky.”

While investigators continue building a picture of what happened on Sunday, sources have said it is possible the sensitive detonator went off accidentally, killing Swealmeen.

“In moving to get ready to get out of the taxi, he might have set it off. Or else, as the vehicle has come to a stop, he has dropped it or jostled it and it has gone off,” said a source. Home-made devices are inherently unstable.

Police have uncovered at least two types of explosives in Swealmeen’s flat, and chemicals purchased online suggest he had built a sophisticated detonator intended to set off a much larger bomb. 

On Wednesday, residents living close to the property in Sutcliffe Street, in the Kensington area of Liverpool, were evacuated when police found “several suspicious packages”.

Evidence uncovered so far suggests Swealmeen’s device was smaller than the bomb constructed by Salman Abedi, the Manchester Arena suicide bomber who murdered 22 people and wounded hundreds more. 

One likelihood is that the bomb was being constructed using the HMTD high explosive, similar to that used in the 7/7 attacks in London in 2005.

A source said it appeared Swealmeen had built a primer and a secondary explosive, using two different chemical explosives purchased on the internet. 

Intelligence agencies are scouring for any associates who might also be involved but currently believe he was acting alone and in all likelihood found plans to construct the bomb online.