Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: (CNN) Khuram Shahzad Butt, a 27-year-old British national born in Pakistan who died in a hail of police bullets Saturday night after the London attack, was a tall, lanky, straggly bearded young man of few words -- a brainwashed follower of al-Muhajiroun, a loose grouping of British extremists supportive of ISIS linked to a large number of terrorist plots involving UK nationals. The latest on London attack CNN's Sandi Sidhu met him several times while reporting on the group in the United Kingdom between 2014 and 2016. He went by the name "Abz" or "Abu Zaitun" and seldom spoke during meetings. He was content to leave the task of proselytizing to louder, older members of the group, according to Sidhu.London attacker Khuram Shahzad Butt appeared in a 2016 Channel 4 documentary called "The Jihadis Next Door"At the time of the attack, Butt worked as a receptionist at a fitness center in Ilford, a friend from the gym said 
 Question:The identification of Butt as one of the attackers raised many questions about what _ authorities knew about him and when.

British