An inquiry into the 2017 Manchester Arena suicide bombing, which targeted fans leaving an Ariana Grande concert, found that British intelligence failed to act on information that may have prevented the attack. The bombing, which was carried out by 22-year-old Salman Abedi, left 22 dead and hundreds wounded. Many of the victims were young children and teens.

Retired John Saunders, who led the inquiry, announced on Thursday that an MI5 officer received intelligence on Abedi, but failed to act on it in a timely manner, stating that it was a “significant missed opportunity to take action that might have prevented the attack.”

Saunders concluded that information received in the months leading up to the bombing may have led authorities to arrest Abedi in the days before the attack when he returned from Libya, where he received explosives training by Islamic State affiliated militants. Abedi was also previously known by authorities for his extremist views, but was deemed a low threat risk and his case was closed.

In previous inquiries, Saunders also criticized local police and arena security for not identifying Abedi as a threat at the concert before the blast. In one instance, a security guard refrained from approaching Abedi over fears he would be called racist.

In a televised statement, MI5 Director General Ken McCallum apologized for MI5’s failure to prevent the attack, saying that “Gathering covert intelligence is difficult, but had we managed to seize the slim chance we had, those impacted might not have experienced such appalling loss and trauma.”