While the immediate and urgent focus of the emergency services after the stabbings in south London will be the victims, the police and MI5 will swiftly be reviewing what they knew about the perpetrator, after the revelation that he was under surveillance at the time of the incident.

Following the police statement saying that officers were present “as part of a proactive Counter Terrorism operation” there will inevitably be questions about why the man – now named as 20-year-old Sudesh Amman – was under surveillance, for how long, and whether officers had chances to intervene before he launched his attack.

A Whitehall source said: “He was under surveillance, that is what allowed police to do their job so quickly. It could have been much worse than it was.”

Putting someone under surveillance is not done lightly, and for good reason. It would require a senior officer in the Metropolitan police to authorise “directed” surveillance in a public place, or possibly the home secretary to sign a warrant to allow more intrusive surveillance of an individual at their home, or their workplace.

We now know that Amman was known to the authorities. He had been freed only days ago after serving half of his sentence of more than three years for the possession and distribution of extremist material. The surveillance could indicate that they might have feared an attack was imminent.

Play Video 1:30 Streatham attack: police shoot man dead after 'terror related' stabbing – video report

Neither the police nor MI5 has the resources to put a large number of people under surveillance at any one time. It is massively resource-intensive, potentially requiring teams of officers working to cover 24-hour shifts; which is why investigators prefer to monitor the communications of people they are worried about, until they believe there is a need to escalate.

It is far too early to know exactly what happened in this case. And is worth remembering that MI5 says it is conducting hundreds of counter-terrorism investigations simultaneously and has 3,000 “subjects of interest”. The agency speaks of having a larger pool of 20,000 people “who have been subjects of interest in previous terrorism investigations”. The task for the agency – and for the counter-terrorism police with whom they work – is complex and involves high stakes.

There is another truth: the more complex a potential terrorist attack, the better the chances are of it being foiled by the police and Britain’s intelligence agencies. The most difficult attacks to predict, and to prevent, involve the “lone wolf”.