Potential terrorists are being "remotely radicalised" after a rapid expansion of online communications platforms, a watchdog has warned.
Max Hill QC, Independent Reviewer of Terrorism Legislation, highlighted the "acute" difficulties in identifying contact between extremists in the digital age.
In his annual report for 2016, published by the Government on Thursday, Mr Hill referred to the flurry of atrocities in Britain last year.
He said: "Whilst we must all wait for the full facts of the 2017 attacks in London and Manchester to emerge, it seems that some of those who committed terrorist murders on our streets may have reached their murderous state having been influenced by what they read and what they see online, just as much as by whom they meet.
"Even where large amounts of extremist material have been consumed, many radicalised individuals still come into contact with one or more radicalisers, who are themselves often using online platforms.
"It is this element of 'remote radicalisation' which is acutely difficult to spot."
Mr Hill pointed out that until a few years ago "radicalisers" would "suborn" young men in person and "brainwash them into a plan for action".
"That still goes on," he said. "But we are now seeing something comparatively new, running alongside."
Discussing the changing nature of the threat, he referred to a "rapid and recent expansion in online communications platforms, which are now used by terrorists".
The report went on: "Not so many years ago, those planning terrorist attacks were still using text messages or Blackberries, they were meeting in person in each other's homes, in local open spaces, and during shopping trips for the everyday items they needed to make the IEDs (improvised explosive device) they planned to deploy.
"And, equally important, there would usually be clear influence exerted over would-be terrorists by radicalisers or trainers, those who spent time with their acolytes inspiring them to take life and even to end their own life in so doing."
The independent reviewer noted that where terrorism was "facilitated" by the use of social media, there was a desire to "close down the criminals' ability to communicate".
But, he said, it must be recognised that policing the internet and controlling social media comes "at a very high price" if it interferes with the freedom of communication every citizen enjoys.
Mr Hill's report added: "This is uncertain territory. Driving material, however offensive, from open availability into underground spaces online would be counter-productive if would-be terrorists could still access it.
"And once this material goes underground, it is harder for law enforcement to detect and much harder for good people to argue against it, to show how wrong the radical propaganda really is."
Pressure on technology firms over their role in helping confront the terror threat intensified last year as Britain was hit by five attacks.
In October the head of MI5 Andrew Parker described how plots were escalating to the point of violence in a matter of days.
Mr Parker flagged up how extremists could exploit "safe spaces" online, making the task of disrupting their activities more difficult.
Elsewhere in his report, Mr Hill suggested that "careful consideration" be given to the "existence of any ongoing need" for a number of offences in terrorism legislation which he said were now used rarely, if at all.
They included sections relating to terror training and inciting terrorism overseas.
The assessment also recommended that the Joint Terrorism Analysis Centre, which sets the official threat level for international terrorism, should in future consider activity including domestic extremism.