The police were alerted by a 999 call as the attack was in its 51st second, just after Masood rushed out of his vehicle and headed toward the Houses of Parliament.
The Westminster terrorist was previously known to security services and had once been investigated by MI5 over violent extremism.
Masood plowed a rented SUV into pedestrians on London's Westminster Bridge, killing two and injuring more than 30 people of nearly a dozen nationalities. Two people are still in critical condition and one has life-threatening injuries.
"If you knew Khalid Masood and had spoken to him in the months, weeks or days leading up to this attack please come forward".
Commentators pointed out the group had a record of opportunistically claiming attacks and it was significant the statement did not appear to claim it had directed the strike.
This brings the total number of victims to four, including a police officer and excluding the attacker, who was shot dead outside Parliament. The Islamic State group has claimed responsibility for the attack, calling him a "solider" who responded to its demands that followers attack countries in the coalition fighting IS in Syria and Iraq. "There might be people out there who did have concerns about Masood but did not feel comfortable for whatever reason in passing those concerns to us". The footage shows pedestrians walking by the open gates and even a courier entering the grounds.
The new footage follows the release of earlier video showing slight delays and confusion during the evacuation of Prime Minister Theresa May from Parliament as the attack unfolded.
Masood carried out the deadliest in Britain since the London suicide bombings of 2005 by four young British Islamists, which killed 52.
One victim, Danny Smith, told The Sun newspaper that Masood had stabbed him in the face with a kitchen knife after an argument just three days after they met. Authorities haven't charged or identified him.
Basu acknowledged that while the detectives of the Metropolitan Police's Counter Terrorism Command are investigating "at pace" trying to "establish with absolute clarity why he did these unspeakable acts", the answer may not be found at all.
The dead were identified as Kurt Cochran, 54, of Utah and British school administrator Aysha Frade, 43 - both struck on the bridge - and 48-year-old Constable Keith Palmer, a 15-year veteran of the Metropolitan Police.
On Friday, Rowley thanked journalists who agreed to police requests not to publish Masood's name while investigators were still taking stock of the potential threat and arranging arrests and searches.
Masood, 52, was born in Kent to the southeast of London and moved through several addresses in England, although he was known to have lived recently in Birmingham in central England. [J] a 35-year-old-man has been released with no further police action; [K] a 32-year-old woman has been bailed pending further enquiries to a date in late March.
Details about how he became radicalized aren't clear, although he may have become exposed to radical views while an inmate in Britain or while working in conservative Saudi Arabia.