British police have cast their nets wider and made two "significant" arrests yesterday, as detectives race to establish if Wednesday's attack on the United Kingdom's Houses of Parliament was part of a larger terror conspiracy.
Nine people are in custody after security forces conducted raids in the wake of the deadly strike in London that killed four people and injured as many as 50. Two people are still in critical condition in hospital, with one of them suffering from life-threatening injuries.
Security forces made overnight raids in the West Midlands and Manchester, where a 35-year-old man was reportedly nabbed in one of the city's most affluent suburbs.
The nine were arrested "on suspicion of preparation of terrorist acts", said the police, who yesterday appealed to the public to share information about the attacker or any "associates".
British Prime Minister Theresa May told lawmakers in Parliament on Thursday that British-born Khalid Masood, 52, was believed to have acted alone when he drove a rented SUV into pedestrians on the busy Westminster Bridge, then stabbed a police officer at the gate of the Parliament grounds.
Since the attack three days ago, investigators have interviewed as many as 3,500 witnesses, of whom 1,000 were from Westminster Bridge and the rest from Parliament.
The police also seized 2,700 items as part of their investigation, code-named Operation Classific and led by SO15, Scotland Yard's counter-terrorism command.
The police's head of counter-terrorism Mark Rowley said yesterday that the probe is focused on uncovering Masood's motivation and preparation, and finding out whether he had associates.
"Our determination is to find out if he acted totally alone, inspired by terrorist propaganda, or if others encouraged or supported him," he said.
Security remains heightened throughout the capital city.
Mr Rowley said there would be an "enhanced" armed and unarmed presence in London over the next few days.
Other cities across Britain have also strengthened armed patrols in the wake of the attack.
Security arrangements around the Houses of Parliament will also be reviewed.
"Our current arrangements have been developed with Parliament over many years and are designed to provide access to the seat of our government, balanced carefully with security that is proportionate but not overly intrusive," Mr Rowley told reporters yesterday.
"My team will work with the parliamentary authorities to assess whether different tone or balance is necessary."
Armed with two knives, Masood had run about 30m from where he crashed his car, to Carriage Gates, the entrance to the Houses of Parliament which MPs use to access an underground carpark.
British media yesterday reported that Masood managed to get past the gates, which were open as Acting Metropolitan Police Commissioner Craig Mackey was leaving the estate in a car after a security meeting. Masood then stabbed police officer Keith Palmer to death.
Three other people died after Masood's car ploughed into them on Westminster Bridge: Spanish teacher Aysha Frade, 43; American tourist Kurt Cochran, 54; and retired window cleaner Leslie Rhodes, 75, who succumbed to his injuries late on Thursday.
Faith leaders held a vigil for them at Westminster Abbey yesterday, as Mrs May and Prince Charles separately visited people wounded in the attack as well as medical staff at the hospitals.
The Romanian tourist who fell into the River Thames during the terror blitz is in a stable condition, the Romanian ambassador told BBC News.
The injured woman, 29-year-old architect Andreea Cristea, is still unconscious, but doctors have removed a blood clot from her brain, said the ambassador.