Two women and four men arrested in the central England city of Birmingham, where London terror attacker Khalid Masood recently lived, have been released.

British police say six of the 10 people being held in connection with the attack near Britain’s Parliament have been released without charge.

The two women and four men were all arrested in the central England city of Birmingham, where attacker Khalid Masood recently lived. All had been arrested on suspicion of preparing terrorist acts.

Four other people remain in custody as police try to learn how Masood became radicalised and went on a rampage Wednesday in London with an SUV and knives that killed four people.

He was later shot to death by police.

Police believe Masood acted alone but want to know whether others helped him prepare the attack.

Metropolitan Police counter-terrorism chief Mark Rowley said the police investigation was focusing on understanding his motivation, his operation and his associates.

“Whilst there is still no evidence of further threats, you’ll understand our determination is to find out if either he acted totally alone, inspired perhaps by terrorist propaganda, or if others have encouraged, supported or directed him,” Mr Rowley said.

Police are combing through “massive amounts of computer data” and more than 2,500 items seized in searches.

“We’ve seized 2,700 items from these searches, including massive amounts of computer data for us to work through,” Rowley said, adding that contact had been made with 3,500 witnesses.

Masood, who  had shown up on the periphery of previous terrorism investigations that brought him to the attention of Britain’s MI5 spy agency.

But he was not under investigation when he sped across Westminster Bridge on Wednesday, ploughing down pedestrians with a hired car before running into the parliamentary grounds and fatally stabbing an unarmed policeman. He was shot dead by police.

Masood was once investigated in relation to concerns about violent extremism by British authorities, but the Metropolitan Police have said he “was not the subject of any current investigations and there was no prior intelligence about his intent to mount a terrorist attack”.

“He was a peripheral figure. The case is historic — he was not part of the current intelligence picture. There was no prior intelligence of his intent — or of the plot,” British Prime Minister, Theresa May told Parliament.

Mr Rowley said at least 50 people were injured in the attack.

He named 75-year-old Lesley Rhodes as the fourth victim, who died in hospital on Thursday.

Two remain in hospital in a critical condition, one with life-threatening injuries.

Scotland Yard said they have seized close to 3,000 items in raids across the country, including what is thought to be critical computer data.

— with agencies