London attack ringleader was intensely investigated, starting 2015

London attack ringleader Khuram Butt was identified as a major potential threat, which led to an investigation starting in 2015.

"The British police and the MI5 began investigating Butt intensively as part of a major drive to dismantle and destabilise al-Muhajiroun, a grouping of extremists in the U.K. supportive of the ISIS," U.K. counter-terrorism officials told CNN.

The investigation into Butt involved a "full package" of investigatory measures.

As soon as the British extremists began to flock to Syria to join the ISIS' newly declared caliphate in 2014, counter-terrorism officials moved more aggressively to take down al-Muhajiroun.

Butt was considered to be potentially one of the most dangerous extremists in the U.K., owing to his hardline views.

After the ISIS called for attacks in the West in September 2014, the British security services grew increasingly concerned that al-Muhajiroun members, who had remained in the U.K., would carry out terrorist attacks. One of those they were most concerned about was Butt.

The police say Butt was one of the three men, who rammed a van into pedestrians on the London Bridge before launching a stabbing spree in bars and restaurants at nearby Borough Market on Saturday night. All three were shot dead by the police.

As many as seven people were killed and 48 others were injured.