Metropolitan Police Acting Deputy Commissioner and Head of Counter Terrorism Mark Rowley said in a statement that a total of nine persons remain in custody and one woman has been released on bail.
Scotland Yard also conceded that they may "never understand" the 52-year-old attacker's motive.
The Scotland Yard's Indian-origin deputy assistant commissioner, Neil Basu, in a statement said Khalid Masood's attack had echoes of the rhetoric of the Islamic State but no evidence at this stage suggests he was linked to the group.
Who were they? What led them to mass murder?