So-called Islamic State has claimed it was behind the Westminster terror attack which left four people dead, according to the IS-affiliated new agency Amaq.
In a statement the terror organisation said: "The attacker yesterday in front of the British Parliament in London was a soldier of the Islamic State executing the operation in response to calls to target citizens of coalition nations."
Professor Peter Neumann, director of the International Centre For The Study Of Radicalisation (ICSR) at King's College London, said: "You can be a lone attacker but you can still be radicalised as part of networks as part of, for example, a jihadist scene."
Dr Shiraz Maher, deputy director of the ICSR, said the West is seeing an increasing number of terrorist attacks using "unsophisticated methods".
He said: "These are plots that are very easy to construct, require little money, planning, and expertise, but which are nonetheless highly effective in causing death and destruction.
"We have already seen similar attacks in Nice and Florida, and the challenge for all of us is to now work out how we can identify and prevent these attacks before they occur."
All those arrested in connection with the Westminster attack have now been released by police.
Tony Davis was the first civilian to reach Pc Palmer after he was stabbed during the Westminster attack.
Defiant Londoners put on a show of solidarity for the victims of the Westminster terror attack at a vigil in their honour.