Birmingham, England (CNN) Police investigating the deadly terrorist attack in central London are focusing closely on how the perpetrator became radicalized, as they battle to piece together his identity and connections.

Britain's most senior counter-terror police officer, Mark Rowley, described the inquiry as "very large and fast-paced," and announced two "significant arrests" Friday. Hundreds of officers had made contact with thousands of witnesses, he said.

Authorities had earlier portrayed the attacker, 52-year-old British man Khalid Masood , as having acted alone. It was unclear whether the arrests indicated that Masood may have had associates.

Masood is known to have used multiple aliases during his life. He was born Adrian Russell Ajao but also used the name Adrian Elms, police said.

Asked about his possible conversion to Islam and subsequent radicalization, Rowley said: "Clearly that's the main line of our investigation, is what led him to be radicalized, was it through influences in a community, influences from overseas, or through online propaganda."

He appealed for anyone who knew Masood well or was aware of his recent movements to get in touch, as detectives probe his motivation, his preparation for the attack and any associates.

Arrests and raids

Of 11 people arrested so far, two remained in custody as of late Friday, London's Metropolitan Police said. The two in custody are a 58-year-old man and a 27-year-old man, police said. Both men were arrested Thursday in Birmingham, according to police.

All but one of the people brought in were detained on suspicion of preparation of terrorist acts.

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Arrests were made in London, Birmingham, central England, and Manchester, north west England, police said. Addresses were searched in Brighton, on the south coast of England, Surrey, to the west of London, and in the county of Carmarthenshire, in Wales. Two searches continue in Birmingham and one in London.

Officers have seized 2,700 items, including "massive amounts" of computer data, and have had contact with 3,500 witnesses to the attack, many of them of different nationalities, Rowley said.

The number of armed officers remains at nearly double strength in London in the wake of the attack, Rowley said, while extra police officers are also on patrol elsewhere in the UK.

Details about killer

Details are gradually emerging about Masood, who was shot dead by a police firearms officer after he rammed a car into pedestrians on Westminster Bridge and stabbed an unarmed police officer outside parliament.

Police searched a property in the Winson Green area of Birmingham, where Masood is thought to have been living with his family until last December. Police could be seen Friday carrying boxes from the property, a modern, redbrick town house in a quiet cul-de-sac.

Marjoli Gajecka, 26, told CNN she had seen Masood and his family often when she visited her mother, who lives on the street. Masood kept to himself, she said, adding: "He was never really talkative with neighbors."

She said Masood had two children, a boy and a girl, whose ages she estimated at six or seven.

She was "very shocked" by the news he had carried out the attack, she said, "because they just seemed a normal family, a normal Muslim family."

Police raided an apartment above this Persian restaurant.

Gajecka said she saw Masood and his family moving out of the house around Christmas time. They are thought to have relocated to a shabby looking flat above a Persian restaurant on busy Hagley Road, which was also raided by police Thursday.

Masood stayed in a $75-a-night hotel in Brighton, about 55 miles south of London, for several nights before the attack, staff said.

The reception manager at the Preston Park Hotel told CNN that he came across "as an absolutely normal client." The employee, who did not give her name, said she saw the name Masood on his credit card. She said the hotel didn't asked for his ID, in line with its normal practice.

The Preston Park Hotel in Brighton, where Masood stayed before the attack.

Born in Kent, Masood had previous convictions, including some for violent offenses but none for terrorism, police said. His most recent conviction was in 2003 for possession of a knife.

Asked about reports Masood worked as an English language teacher, Britain's Department for Education said that according to their records Masood was not a teacher in any English language school. Because of his criminal record, he would not pass the necessary checks to become a qualified teacher.

Photos: Deadly attack outside UK Parliament A car is seen crashed into a fence outside the Parliament building in London on Wednesday, March 22. Police have launched a "full counter-terrorism investigation" after an attacker rammed a car into crowds of people and stabbed a police officer on Parliament grounds. Hide Caption 1 of 15 Photos: Deadly attack outside UK Parliament An armed police officer stands over the attacker, identified as Khalid Masood, outside Parliament. "It appeared that a car was coming towards the House of Commons mowing down pedestrians on the way," Member of Parliament Gerald Howarth told CNN. "The driver then got access to the parliamentary estate, stabbed a police officer and was shot." Hide Caption 2 of 15 Photos: Deadly attack outside UK Parliament Masood is treated by emergency services as police look on at the scene outside the Houses of Parliament. The Metropolitan Police say he was born in Kent, but is believed to have been living in the West Midlands recently. Police say Masood was also known by a number of aliases. "Masood was not the subject of any current investigations and there was no prior intelligence about his intent to mount a terrorist attack," Met Police said in a statement. Hide Caption 3 of 15 Photos: Deadly attack outside UK Parliament Member of Parliament Tobias Ellwood, in the glasses, tends to one of the injured people amid the chaos. The man the politician was trying to save was a police officer who died, a witness on the scene told CNN. Authorities identified the deceased officer as Keith Palmer, 48. Hide Caption 4 of 15 Photos: Deadly attack outside UK Parliament Emergency workers attend to injured people at the scene. Hide Caption 5 of 15 Photos: Deadly attack outside UK Parliament Medics treat a victim on Westminster Bridge. Hide Caption 6 of 15 Photos: Deadly attack outside UK Parliament People leave the Parliament building after the incident. Lawmakers were forced to remain in the main debating chamber of the House of Commons as police responded to the incident outside. Hide Caption 7 of 15 Photos: Deadly attack outside UK Parliament A person is treated at the scene. Hide Caption 8 of 15 Photos: Deadly attack outside UK Parliament Medical aid is provided outside Parliament. Hide Caption 9 of 15 Photos: Deadly attack outside UK Parliament Tourists were trapped for a time in cars on the London Eye Ferris wheel, which was stopped in the immediate aftermath of the attack. Hide Caption 10 of 15 Photos: Deadly attack outside UK Parliament A police officer stands guard near Westminster Bridge and the Houses of Parliament. Hide Caption 11 of 15 Photos: Deadly attack outside UK Parliament A person receives medical treatment at the scene. Hide Caption 12 of 15 Photos: Deadly attack outside UK Parliament At right, the car of British Prime Minister Theresa May is driven away from Parliament. Hide Caption 13 of 15 Photos: Deadly attack outside UK Parliament An air ambulance arrives at the scene. Hide Caption 14 of 15 Photos: Deadly attack outside UK Parliament Armed police enter the Houses of Parliament. Hide Caption 15 of 15

Attack victims

The attack left four people dead and injured at least 50 more, 31 of whom needed hospital treatment, Rowley said. Two people are in critical condition and another has life-threatening injuries.

Two police officers who were injured as the attacker plowed through crowds walking across Westminster Bridge also remain in hospital with "very significant" injuries, Rowley said.

The fourth victim, a 75-year-old man, died Thursday night after his life support was withdrawn, according to London's Metropolitan Police. He was named by Rowley as Leslie Rhodes, from Streatham in south London.

The other three who died were policeman Keith Palmer, stabbed as he protected Parliament, American tourist Kurt Cochran and college administrator Aysha Frade, a British citizen with Spanish roots.

The Romanian Foreign Ministry said a Romanian woman remained in critical condition after undergoing complicated surgery for her injuries.

Two members of Parliament have been honored by Queen Elizabeth II for their heroics in the attack, Downing Street announced Friday.

MP Tobias Ellwood, who tried to save the life of Palmer, and Security Minister Ben Wallace, who helped coordinate the government's response, have been approved as members of the queen's Privy Council.

Ellwood and Wallace will now be entitled to be referred to as "right honorable" as members of the group of advisers to the monarchy that includes present or former members of the House of Commons or House of Lords.

Vigils and defiance

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"We come together as Londoners tonight to remember those who have lost their lives and all those affected by the horrific attack yesterday. But also to send a clear message, Londoners will never be cowed by terrorism," London Mayor Sadiq Khan told the crowd.

Khan laid a wreath Friday at New Scotland Yard in remembrance of Palmer, the slain police office.