There were renewed appeals for unity in a climate of Brexit-related Islamophobic hate crime after Wednesday's attacker was identified as Khalid Masood, while Scotland Yard made two "further significant arrests" and the Westminster village returned to normal on Friday.
British police say six people, including two women and four men, being held in connection with the attack have been released without charge.
Searches have been either completed or are in-process at 21 locations, Rowley said.
Ten people have now been arrested in relation to the attack that left four victims dead and 40 injured. All victims have been named, and two people remain in critical condition.
Many suspects in British terrorist attacks or plots have had roots in Birmingham, England's second-largest city, and several local mosques have been linked to extremist clerics. He had not been the subject of any current police investigations.
Police also carried out two more arrests on Friday, detaining a 35-year-old man and a 32-year-old woman.
Metropolitan Police said they have made contact with some 3,500 witnesses, including 1,000 people who were on or around Westminister Bridge at the time of the attack.
A 58-year-old man and a 27-year-old man were arrested at separate addresses in Birmingham on Thursday, and detained under the Terrorism Act (TACT).
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Four people, including a police officer, died when Khalid Mosood drove a rented auto into crowds on Westminster Bridge and then stabbed the unarmed police officer before being shot dead. Toumi said Masood checked in under his own name and mentioned having a couple of children, as well as troubles with an ailing father.
Islamic State claimed responsibility in a statement issued by its Amaq news agency, but did not name Masood and gave no details.
Deputy Assistant Commissioner Neil Basu said there was no intelligence to suggest further attacks were planned.
The attack was the first mass-casualty terrorist outrage in Britain since 2005 when 52 people died in the July 7 bomb attacks on the London public transportation system.
Looking to ensure security heading into the weekend, police have almost doubled the number of armed officers in London; elsewhere in the United Kingdom, up to a third more armed officers are on duty.
Rowley said that despite the attempt, terrorists targeting London would not succeed.
May set an unyielding tone Thursday, saluting the heroism of police as well as the ordinary actions of everyone in the British capital who went about their lives in the aftermath.
Scotland Yard said Masood, 52, was born in Kent on Christmas Day in 1964.