The deceased 27-year-old drove a van into pedestrians with two accomplices Rachid Redouane, 30, and Youssef Zaghba, 22, before stabbing people to death in central London on June 3.
They murdered seven people and injured dozens more.
Butt and the two other terrorists were shot dead.
Butt became radicalised after watching hate preachers on YouTube before becoming linked to al-Muhajiroun, the banned extremist group whose leader, Anjem Choudary, is in jail.
It is believed that he moved to the UK from Pakistan when he was a child.
Butt first came to public attention on a Channel 4 programme called The Jihadis Next Door in January last year when he made his extremist views clear.
Chief executive of the Ramadhan Foundation, Mohammed Shafiq, told the Express: "Many of us in the British Muslim community have been demanding action against these extremists to no avail.
"I am not surprised that Khuram Butt carried out the terrorist attack and there are serious questions for the authorities."