Now Isil claims that it inspired attacker to carry out atrocity
Isil's carefully worded claim of responsibility suggested the Westminster attacker was inspired - rather than orchestrated - by the jihadist group.
A statement released yesterday by the official Amaq media agency of Islamic State of Iraq and the Levant (Isil) described the assailant as a "soldier of the caliphate" but gave no other details.
Isil channels on the Telegram app, an encrypted messaging application, shared pictures of a man wrongly accused of being the attacker, fuelling speculation that the group did not actually know who was behind Wednesday's murders.
The Telegram app is favoured by jihadists because governments are unable to hack into its communications.
Isil almost never claims attacks for which it is not in some way responsible.
Some killers have simply pledged allegiance to the group and acted on its general call to arms, while others have had guidance on everything from the target to the method and the timing.
In the case of the Istanbul nightclub bombing on New Year's Eve, the assailant was coached by Isil handlers, who funded and helped plan every stage of the attack.
In other instances, such as the shooting at the Pulse nightclub in Florida last summer, the connection was much more tenuous.
Khalid Masood appeared to be acting on his own, but experts say even where attacks appear to have been perpetrated by "lone wolves", they are rarely acting in complete isolation.
"Even the inspired attacks have tended to occur within the context of a network - in other words, accomplices, which means by definition they are not 'lone'," said Kyle Orton, Middle East analyst at The Henry Jackson Society think-tank.
The downplaying of Isil's direct role in the European attacks had deadly consequences last year when the connections between a series of plots in France were missed, and the diversion of the security forces helped allow the co-ordinated attacks in Paris of November 13, 2015, when suicide bombers and gunmen targeted the Stade de France, restaurants, and the Bataclan concert hall, leaving 130 people dead.
Thirty-one of the 38 Isil- related European plots have involved either online or in- person direction by Isil.
It is unclear whether Masood had travelled to Iraq or Syria, but most attacks carried out in Isil's name have been by home-grown terrorists. (© Daily Telegraph, London)