Police chiefs on Friday appealed for help from the public in finding out how the attacker — born Adrian Russell Ajao in the suburban English county of Kent — became radicalized.

In the hunt for clues, police made two "significant arrests" overnight, bringing the total in custody since Wednesday's attack to nine.

"Clearly a main line in our investigation is what led him to be radicalized," Metropolitan Police Acting Deputy Commissioner Mark Rowley told reporters. "Was it through influences in our community, influences from overseas or from online propaganda? Our investigation and arrests will help in that but the public appeal will make a real difference."

NBC News