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.
Butt, who lived in Barking in east London, appeared several times in a 2016 Channel 4 documentary called "The Jihadis Next Door," which profiled a group of individuals linked to al-Muhajiroun in the United Kingdom. At one point in the documentary, he can be seen helping unfurl a black banner in Regent's Park after a radical preacher promised it would fly one day over 10 Downing Street. The flag was different in form to the one most commonly used by ISIS.
A friend of Butt's identified him to CNN as appearing in the documentary. Neighbors living beside the residence raided by police in Barking also identified Butt from a still frame in the documentary.
Another of the men featured in the documentary was Abu Rumaysah, a British man some have speculated was a masked militant in a January 2016 ISIS execution video. Rumaysah rented out bouncy castles in east London, before traveling to Syria in autumn 2014 after skipping bail. He had been arrested on suspicion of encouraging terrorism and being a member of al-Muhajiroun.
Sidhu observed Butt and Rumayah standing together at al-Muhajiroun protests in 2014. She recalled that Butt seemed particularly close to Abu Rumaysah. Butt helped Abu Rumayash take pictures and film protests organized by the group.
At the time of the attack, Butt worked as a receptionist at a fitness center in Ilford, a friend from the gym told CNN. The gym is described as a mixed martial arts gym on its website. Butt told the friend he had stopped attending al-Muhajiroun meetings some time ago.
The friend said Butt told him his wife had given birth several months ago.
Al-Muhajiroun has been linked to half of all terror plots by British nationals in the United Kingdom and overseas over the last two decades, according to research published in 2015 by Raffaello Pantucci, a terrorism analyst at the Royal United Services Institute. Several followers of the group joined ISIS in Syria including Abu Rumaysah and Abu Rahin Aziz, a Luton, England, resident killed in a US drone strike in Raqqa, Syria, in July 2015.
Al-Muhajiroun's longtime UK leader Anjem Choudary was convicted and sentenced to five years in prison in 2016 for calling on Muslims in the United Kingdom to support ISIS.
In the years after 9/11 the group operated under a variety of guises including Shariah4UK and Muslims against Crusades. Plots involving the group's followers include an attack on a Tel Aviv nightclub in 2003, an al Qaeda orchestrated plot to bomb the United Kingdom with fertilizer bombs in 2004, a plot to bomb a territorial army center in Luton in 2012 and the murder of British soldier Lee Rigby in east London in 2013.
More recently, Brusthom Ziamani, a south London teenager brainwashed by al-Muhajiroun, was convicted in 2015 of plotting to behead a British soldier.
The identification of Butt as one of the attackers raised many questions about what British authorities knew about him and when. Mohammed Shafiq, chief executive of the Ramadhan Foundation, said that after confronting Anjem Choudary for his support for terrorism near the Houses of Parliament a day after Lee Rigby's killing he was called a "traitor" by the future London attacker, Butt.
"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," Shafiq stated.