https://www.bbc.co.uk/news/uk-england-manchester-64815723?at_medium=RSS&at_campaign=KARANGAThe head of MI5 said he was "profoundly sorry" the security service did not prevent the Manchester Arena attack.
A public inquiry found MI5 missed a significant chance to take action that might have stopped the 2017 bombing.
Chairman Sir John Saunders said the intelligence could have led to suicide bomber Salman Abedi being followed to a car where he stored his explosives.
MI5 director-general Ken McCallum said he regretted that such intelligence was not obtained.
"Gathering covert intelligence is difficult," he said, "but had we managed to seize the slim chance we had, those impacted might not have experienced such appalling loss and trauma."
Twenty-two people died and hundreds were injured when Abedi detonated his homemade device in the foyer of Manchester Arena as crowds left an Ariana Grande concert on 22 May 2017.
The inquiry found two pieces of information about Abedi were assessed at the time by the security service as not being terrorism-related.
An officer admitted they considered a possible pressing national security concern on one of them but did not immediately discuss it with colleagues and did not write up a report that same day.