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The Prime Minister said more needed to be done to tackle online radicalisation, including dealing with increasingly sophisticated efforts to avoid detection.
And he said the authorities had to get better at “joining the dots”, bringing together pieces of evidence which on their own may not amount to warning signs but together were a signal of a possible threat.
A series of attackers in recent years have been on the radar of the authorities but had not been prioritised.
Manchester Arena bomber Salman Abedi was categorised as a “closed subject of interest” by MI5 at the time of the 2017 attack, and so not under active investigation.
In 2013, Fusilier Lee Rigby was run over and stabbed by Michael Adebolajo and Michael Adebowale in south-east London.
Both men had previously been investigated by MI5.
We need to be much more ruthless in joining the dots together
The 2017 Westminster Bridge attacker Khalid Masood who mowed down pedestrians and stabbed Pc Keith Palmer before being shot by police in the grounds of Parliament, was also known to the Security Service.
The Prime Minister told MPs on the Liaison Committee: “We need to be much more ruthless in joining the dots together.
“Because so often with characters who are responsible for terrorist attacks… there will be things in their past that are signals or indicators of a likelihood that they will do this.
“Each of them individually may not be enough to trigger our concerns but we need to bring them together.”
Mr Johnson called for online firms to take more responsibility for what appears on their platforms.
“I think there’s always much more that we should do to tackle radicalisation, online or otherwise,” the Prime Minister said.
“When you look at some of the cases that we have seen recently, it’s clear that people are increasingly adept at using devices to conceal their internet tracking, history, as they radicalise themselves.
“We need to be wise to that.”