Would it have actually been a surprise if they were not responsible?

The extremist group's claim of the deadly van attack in Barcelona was made on their news agency Amaq. 

A message read: "The perpetrators of the attack in Barcelona are Islamic State soldiers and carried out the operation on command of Khilafah of targeting coalition countries". 

The "coalition" refers to the US-led force fighting Isis in Iraq and Syria. 

Isis often quickly claim responsibility for terrorist attacks in Europe and elsewhere, even if they have been carried out by so-called 'lone wolves'.