London's Metropolitan Police have made two further "significant" arrests in connection with the attacks on Westminster Bridge and Parliament Wednesday that left five people dead and 50 people injured.
In a news conference Friday, Acting Deputy Commissioner Mark Rowley appealed to the public that if anyone knew the attacker, identified Thursday as Khalid Masood, his associates or recent movements, they should contact the police.
The new arrests were made Thursday night in the West Midlands and north-west England. Nine people remain in custody and 2,700 pieces of evidence have been seized.
The casualty numbers from the attacks were also revised with now more than 50 people injured, two in a critical condition. The death toll from the attacks rose to five overnight, with 75-year-old Leslie Rhodes from south-west London dying from his injuries.
Rowley said London would remain at "double strength" in terms of the number of police officers on the streets and stressed this would be in order to protect and reassure the public in the days after the attack.
On Wednesday afternoon, the suspected terrorist drove a car at speed along the pavement of Westminster Bridge, ramming pedestrians before crashing into a fence near Parliament.
The assailant then ran towards the building which houses U.K. lawmakers and stabbed a police officer before being shot.
U.K. Prime Minister Theresa May condemned London's worst terror attack in over a decade as "sick and depraved".
Britain's 2005 terror attack saw 52 people killed by Islamist suicide bombers on London's transport network.
Islamic State claimed responsibility for Wednesday's terror attack, according to a statement from its Amaq news agency, however it remained unclear whether the assailant was directly connected to the group.
Police confirmed Masood was born in south-east England and had been living in the West Midlands region until Wednesday.
"Masood was not the subject of any current investigations and there was no prior intelligence about his intent to mount a terrorist attack," Rowley said Friday.
"However, he was known to police and has a range of previous convictions for assaults, including GBH (grievous bodily harm), possession of offensive weapons and public order offences."
The day after the attack, several thousand people gathered in London's Trafalgar Square to attend a vigil and remember the people who had lost their lives. London Mayor Sadiq Khan told the crowd, "Londoners will never be cowed by terrorism."