London Metropolitan Police said Thursday that their investigation into yesterday's deadly terrorist attack outside the Palace of Westminster in central London that left four people dead and forty more injured would focus on the attacker's "motivation and preparation".

In an official statement outside New Scotland Yard HQ in London, Acting Deputy Commissioner Mark Rowley said that six addresses around the country had been searched, including during an overnight raid on a house in Birmingham.

The police at 10:16 am GMT said that eight arrests had been made. An initial update from Rowley said that seven people were in custody. 

We have now made a total of eight arrests as part of the ongoing Counter Terrorism operation #WestminsterAttack

— Metropolitan Police (@metpoliceuk) March 23, 2017

"Greater clarity is now developing regarding the casualty figures as we have now collated information from the public and five hospitals - the latest figures I have are that there are currently four dead and 29 people were treated in hospital," Rowley said. "We are still collating numbers of walking wounded and of those in hospital sadly 7 of them are in a critical condition."

Officials also said the attacker has likely been identified, but are not prepared to name him, his nationality or his potential associates other than to say they are considering the attack as an incident of Islamic-inspired international terrorism. 

"I would continue to ask the media not to identify the attacker whilst we are at a sensitive stage in our investigation" #WestminsterAttack

— Metropolitan Police (@metpoliceuk) March 23, 2017

"To be explicit - we have no specific information about further threats to the public at this time" #WestminsterAttack #London

— Metropolitan Police (@metpoliceuk) March 23, 2017

Britain's Houses of Parliament, which sits inside the Palace of Westminster, will open as usual Thursday, lawmakers have said, following a defiant statement last night from Prime Minister Theresa May. 

"The location of this attack was no accident. The terrorists chose to strike at the heart of our capital city, where people of all nationalities, religions and cultures come together to celebrate the values of liberty, democracy and freedom of speech," May said in a nationally-televised address. "These streets of Westminster - home to the world's oldest parliament - are ingrained with a spirit of freedom that echoes in some of the furthest corners of the globe."

May is also due to address lawmakers later Thursday in the House, which remains under heavy police and security services protection following Thursday's attack.

Police have also confirmed that name of the officer murdered in yesterday's attack as 48-year old PC Keith Palmer, who died after receiving multiple stab wounds from the attacker, who appears to have driven his car at speed along Westminster Bridge, which runs adjacent to Parliament, striking several pedestrians -- two of whom later died -- before lunging at PC Palmer with two large knives.

Four people -- including the alleged attacker -- are said to have died in the attack and forty more were injured, some of them gravely, after being struck by the speeding car, which reports say was rented by the attacker in the same area of Birmingham that was subject to the Met police search. 

Westminster Bridge -- a busy connection point between north and south London across the River Thames -- remains closed Thursday as police continue the investigation amid what Scotland Yard has described as "large and complicated crime scene" while the city and the country remain on high alert.