Over 250 individuals connected to the 2017 Manchester Arena attack are suing MI5 for alleged negligence.

The attack, during an Ariana Grande concert, resulted in 22 fatalities.

Salman Abedi, identified by MI5 in 2014, executed the bombing; critical information on him was not adequately acted upon.

An official inquiry acknowledged MI5's missed opportunity to thwart the deadliest U.K. extremist attack in recent years.

Victims filed a group claim with the U.K.’s investigatory powers tribunal, and the case is currently active.

A benefit concert by Ariana Grande was organized in June 2017 to support the victims.

MI5's head, Ken McCallum, has publicly expressed regret over the agency's shortcomings in the case.