Hours before Wednesday's attack in London, the head of the European Union police agency Europol warned that a large group of radicalized individuals posed a constant threat to Britain and Europe.

"Some of these are likely to succeed in the future," Rob Wainwright, who is British, wrote in a blog to commemorate the attacks in Brussels that killed 32 people on March 22, 2016.

In the year between those incidents and the attack that killed three people and injured dozens near the British Parliament, European security officials say intelligence sharing on potential threats has increased 10-fold.