These kinds of unsophisticated attacks are impossible to prevent except to identify and act upon people who adhere to the Islamist ideology. That’s difficult in a free society.
The Westminster attacker was British-born and known to the police and intelligence services, Prime Minister Theresa May has revealed.
She told MPs he had been investigated some years ago over violent extremism, but was “peripheral” and was not part of the current intelligence picture.
The so-called Islamic State group has said it was behind the attack.