LONDON - Britain's security chief said on Thursday he deeply regretted that his spies missed a "significant" opportunity to stop a 2017 suicide bomber at the end of an Ariana Grande pop concert in Manchester.

Twenty-two people - the youngest of whom was eight, were killed and more than 200 injured when a man detonated a pipe bomb at the Manchester Arena in northern England as relatives went to pick up their children after an American singer's concert.

John Saunders, chairman of the public inquiry into the tragedy, said he could not say with certainty that the bombing could have been stopped, but "there was a reasonable chance of obtaining useful information that could have led to action to prevent the attack."

“He said the domestic spy agency MI5, whose agents he interviewed in private, did not act quickly enough. Saunders spoke after releasing its third and final report on the deadliest bomb attack in Britain since the London suicide bombings of 2005.

Director General of MI5, Ken McCallum, expressed his "profound regret" that his organization could not stop the attack.

"Gathering covert intelligence is challenging, but if we had been able to take advantage of the small window of opportunity, those affected might not have gone through such horrifying loss and trauma," he said in a statement.

A "significant missed opportunity to take action that might have prevented the attack" had been missed, Saunders stated at a press conference. Because of concerns for national security, he claimed he was unable to provide specifics, acknowledging that this might leave the families of the victims wanting to know more.

Richard Scorer, attorney for the 11 survivors, said the Saunders report revealed "unacceptable" errors. “At the very least, the real possibility of preventing this attack has been lost." "This is a devastating conclusion for us," he said.

Suicide bomber Salman Abedi, 22, known to the security service since 2014, has visited an influential activist in prison and is expected to be placed in a deradicalization program, Saunders said.

Abedi's younger brother Hashem was sentenced to 55 years in prison in 2020 for encouraging and helping him, while his third eldest brother Ismail was sentenced in absentia in July for failing to show up at a hearing to testify about escaping Brittany.

The brothers were born to Libyan parents who fled to Britain during Muammar Gaddafi's reign.

Suella Braverman, the interior minister, said she would work with the agency and police to "do everything possible to prevent a repeat of this horrifying attack."

Saunders' previous two reports had also highlighted other flaws and errors in both the venue's security and the emergency services response, claiming that one victim would have likely survived if the response had not been so flawed.

Some victims' relatives stated that they could never forgive those who had let them down.

"From top to bottom - MI5 to the attacker's associates—we will always believe that you all played a role in the murder of our children," said Caroline Curry, mother of a teen who died alongside his 17-year-old girlfriend.

"A lot of people were paid that night to protect our children, but a lot of people didn't fulfill their duty."