Scotland Yard also conceded that they may "never understand" the 52-year-old attacker's motive.
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.
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.