The eight people arrested after lone attacker Khalid Masood killed four people outside of the British parliament are suspected of preparing terrorist attacks, London police said on Thursday.

In a statement, Scotland Yard officials they were still searching properties in Wales, the central city of Birmingham and east London.

The eight people were arrested in early-morning raids in London and Birmingham, located about 130 miles north of the British capital.

Police said all eight were held on "suspicion of preparation of terrorist acts," but did not elaborate further.

"The investigation by the Met’s Counter Terrorism Command into the terrorist attack on Wednesday continues to develop at a fast pace and involves hundreds of detectives," Scotland Yard said in the release

Those arrested include a 39-year-old woman in east London, and a 21-year-old woman and a 23-year-old man at one address in Birmingham. A 26-year-old woman and three men ages, 28, 27, and 26, were arrested at a separate address in Birmingham.

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A 58-year-old man was also arrested at a third address in the central city of Birmingham for "suspicion of preparation of terrorist acts."

Police said they still believe Masood, 52, acted alone when he drove an SUV into pedestrians on Westminster Bridge before stabbing a police officer on the Parliament’s grounds.

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According to police, Masood was born in Kent, U.K., and they believe he was most recently living in the West Midlands, which includes the city of Birmingham.

Masood was known to authorities and had a range of previous convictions for assaults, including grievous bodily harm, possession of offensive weapons and public order offenses.

ISIS claimed responsibility for the rampage, saying Masood was “an Islamic State soldier” who “carried out the operation in response to calls to target citizens of the coalition.”

Prime Minister Theresa May called the attack “sick and depraved” in a defiant address to Parliament early Thursday.

“An act of terrorism tried to silence our democracy, but today we meet as normal … we are not afraid and our resolve will never waver in the face of terrorism,” she said. “Democracy and the values it entails will always prevail.”

The dead included a British policeman, stabbed repeatedly, an American tourist who was celebrating his 25th wedding anniversary and a school administrator adored in the Spanish town where she spent summer vacations with her family. A 75-year-old victim of the bridge attack died late Thursday after he was taken off life support, police said.

Police said six remain hospitalized: 4 with critical condition and two with life threatening injuries.

London has been a target for terrorism many times over past decades. Just this weekend, hundreds of armed police took part in an exercise simulating a "marauding" terrorist attack on the River Thames.

May underscored that the attack targeted "free people everywhere," and she said she had a response: "You will not defeat us."

"Let this be the message from this House and this nation today: our values will prevail," she said.

The Associated Press contributed to this report.