British police say that two people remain in custody following last week's attack in London as messaging services face criticism for encrypted networks that allow attackers to communicate in secret.
Attacker Khalid Masood is believed to have used the messaging service WhatsApp before running down pedestrians on Westminster Bridge and storming a gate outside Parliament armed with two knives. Four died in the rampage, including a police officer.
Encryption makes it more difficult to know whether Masood was acting with an accomplice. Britain's Home Secretary Amber Rudd wants technology companies to do more to make it possible for security services to have access to such messages.
Police say that a 30-year-old man arrested in Birmingham on Sunday and a 58-year-old man arrested shortly after the attack remain in police custody
