Police are appealing for information from anyone who knew the London terrorist Khalid Masood as the injury toll climbed to 50.
Assistant Deputy Commissioner Mark Rowley said he wanted to know about Masood's 'associates and places he recently visited'.
It comes after the fourth victim of the attack was named as 75-year-old Leslie Rhodes, from Streatham in south London, as one of the three to have been killed by the car on Westminster Bridge.
Police said 50 people were injured in the attack, an increase on the previous estimate of 40, with two still in a critical condition.
Two more 'significant' arrests have been made, police said, bring the total to 10 arrests with nine held in custody over the attack that left four dead, including police officer Keith Palmer.
Masood hired a car from outside Birmingham and used it to drive at high speed along the pavement of Westminster Bridge, hitting dozens of pedestrians before crashing into the fence at the Palace of Westminster.
He then ran through the gate brandishing a knife and stabbed PC Keith Palmer before being shot dead by defence secretary Michael Fallon's bodyguard.
The latest police statement came as more details of Masood's background emerged.
Born in Kent on Christmas Day 1964 to a black father and a white mother, the 52-year-old was originally named Adrian Elms. He first went to prison after an argument with 'racial overtones' that ended with him attacking café owner Piers Mott in a quiet Sussex village.
It is thought he was radicalised while in prison and adopted his Islamic name, Khalid Masood.
Although he previously convicted, he was thought to be more of a thug than a terrorist. Although he was known to MI5 as a 'peripheral' figure in a terror investigation some years ago, he was 'not the subject of any current investigations, Theresa May told MPs on Thursday.
She added: 'There was no prior intelligence about his intent to mount a terrorist attack.'
The other victims were Aysha Frade, a Spanish teacher in her 40s, and US tourist Kurt Cochran, 54.
