"I know when, where and how Masood committed his atrocities, but now I need to know why," Basu said. "Most importantly, so do the victims and families."
'No evidence' Khalid Masood, London attacker, associated with ISIS: Police →
He says Masood was not a "subject of interest" for counterterror police or the intelligence services before last week's attack.
The Latest: London attacker's mother 'saddened and numbed' →