Two men remain in custody over the deadly London attack as encrypted messaging services such as WhatsApp faced criticism for potentially allowing terrorists to communicate undetected.
A 30-year-old man and 58-year-old man were both arrested in Birmingham on Sunday on suspicion of preparation of terrorist acts.
Police have arrested 12 people since Wednesday's attack when British born Khalid Masood killed four people and was himself shot dead.
Officers on Sunday carried out 15 searches at various addresses in London, Brighton, Surrey, Carmarthenshire, Birmingham and Manchester.
Meanwhile British press reports say Masood used WhatsApp seconds before mowing down pedestrians and storming the Westminster parliamentary estate armed with two knives last week.
On encrypted messaging services, Home Secretary Amber Rudd told BBC One's Andrew Marr Show: "It is completely unacceptable, there should be no place for terrorists to hide.
"We need to make sure that organisations like WhatsApp, and there are plenty of others like that, don't provide a secret place for terrorists to communicate with each other.
"It used to be that people would steam-open envelopes or just listen in on phones when they wanted to find out what people were doing, legally, through warrantry.
"But on this situation we need to make sure that our intelligence services have the ability to get into situations like encrypted WhatsApp."
WhatsApp said in response that it had been assisting the police investigation.
Counter-terrorism officers have spent days piecing together what led Masood, 52, to shed his birth name and later unleash carnage on London.
The Muslim convert was shot dead by armed police after fatally knifing PC Keith Palmer in the Palace of Westminster's cobbled forecourt.
