LONDON (AP) — Britain's domestic intelligence agency didn't act swiftly enough on key information and missed a significant opportunity to prevent the suicide bombing that killed 22 people at a 2017 Ariana Grande concert in northwest England, an inquiry found Thursday.

Retired judge John Saunders, who led the inquiry into the Manchester Arena attack, said that one MI5 officer admitted they considered intelligence about suicide bomber Salman Abedi to be a possible national security concern, but didn't discuss it with colleagues quickly enough.

“I have found a significant missed opportunity to take action that might have prevented the attack,” he said.

MI5 Director General Ken McCallum said he was “profoundly sorry that MI5 did not prevent the attack.”

“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,” McCallum said in a statement.

Abedi, 22, set off a knapsack bomb in the arena’s foyer at the end of the May 22, 2017 concert, as thousands of young fans, including children, were leaving the pop star’s show. Abedi died in the explosion.

His brother, Hashem Abedi, was convicted in 2020 of helping to plan and carry out the attack. He was sentenced to life in prison.

Saunders said had the MI5 acted on the intelligence it received, it could have led to Abedi being stopped at Manchester Airport on his return from Libya just four days before the attack.

Richard Scorer, a lawyer representing 11 of the bereaved families, said the report was a “devastating conclusion for us.”

“It is now very clear that there was a failure to properly assess key intelligence about Salman Abedi; a failure to put it into proper context, and — most catastrophic of all — a delay in acting on it,” Scorer said. “The failures exposed in this report are unacceptable."

Multiple MI5 witnesses gave evidence behind closed doors to the inquiry and the intelligence wasn't publicly disclosed.

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.

Saunders also said that authorities failed to refer Abedi to the government's counterterrorism program, known as Prevent.

“I have concluded that there was at least a period during Salman Abedi’s journey to violent extremism when he should have been referred,” he said.

Thursday's report was the third and final one into the attack. Saunders previously criticized the arena's security staff and local police for failing to identify Abedi as a threat. He has also slammed delays and failings in the response of emergency services on the night of the bombing.