Lawyers from three law firms said Sunday they have submitted a group claim on behalf of more than 250 clients to the U.K.’s investigatory powers tribunal. They said they could not provide further details because it was an ongoing legal matter.

Suicide bomber Salman Abedi set up a knapsack bomb in Manchester Arena, in northwestern England, at the end of Grande’s concert on May 22, 2017, as thousands of young fans were leaving. More than 100 people were injured, many of them children and teenagers. Abedi died in the explosion.

An official inquiry reported last year that Britain’s domestic intelligence agency, MI5, didn’t act swiftly enough on key information and missed a significant opportunity to prevent the bombing, the deadliest extremist attack in the U.K. in recent years.

Abedi had been a “subject of interest” to MI5 officials in 2014, but his case was closed shortly after because he was deemed to be low-risk.

The report also found that one MI5 officer admitted they considered intelligence about Abedi to be a possible national security concern, but didn’t discuss it with colleagues quickly enough.

Ken McCallum, the head of MI5, said in a rare televised statement that he was “profoundly sorry” his agency was unable to prevent the attack.