On Monday, it was reported that the FBI had warned their UK counterparts that Manchester suicide bomber Salman Abedi was planning a deadly act of terrorism in the UK months before he carried out the attack on an Ariana Grande concert that left 22 dead.
In January, the FBI informed the UK intelligence agency MI5 that Abedi was part of a North African Daesh cell based in Manchester that was plotting an attack in the UK, a security source told The Mail on Sunday.
MI5 has opened an internal review after Abedi slipped down the list of suspects under investigation, despite the tip-off. The 22-year-old was on a list of 20,000 people known to MI5 but not one of the 3,000 under active investigation for links to terrorism.
"There are hundreds of thousands of bits of intelligence that come into both MI5 and the police anti-terror hotline which have to be investigated. It appears that Abedi was investigated and it didn't appear that he was on the cusp of committing terror attacks," Phillips said.
"The difficulty is the sheer weight of numbers of people that are on this list means that to give all of them enough security reviews is really, really difficult so there will always be mistakes made and I think we have to accept that if we want to live in a free, open and democratic society."
British police briefly suspended intelligence sharing with the US in a row over a series of media leaks from US officials in the aftermath of Abedi's attack. For example, the New York Times published leaked photographs of bomb fragments and the tattered remains of a backpack.
Phillips thinks the leak most likely came from an individual within US law enforcement, and said it "will make very little difference" to the long-term relationship between the intelligence agencies, which remain committed to sharing information in order to counter the threat.
"I think it's really vital – terrorism is a blight on all our countries and all our countries need to share intelligence around terrorism," Phillips said.
"In the UK, we've got 500 people who have been fighting in Syria and Iraq and they've managed to worm their way back to society. Some of those may decide that they want to commit terror acts. What we've seen here is just one person, effectively, albeit working with a group of others, who's decided to kill himself and take others with him."
"There's such big numbers, and Russia, France, Belgium have all worked out recently that we've all the same problems with this and it's probably not going to go away for generations to come, and it's in all of our benefits to work together to reduce this heinous threat of killing innocent children and their parents," Phillips said.
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