Yahoo News Manchester attack: Security services think they know who bomber is
Police and security services believe they know the identity of the man behind the Manchester suicide bombing.
Speaking outside Downing Street after chairing a meeting of the Government's emergency COBRA committee, the Prime Minister said authorities are working to establish whether he was acting alone or as part of a group.
Theresa May said the blast at Manchester Arena, which left 22 people dead, was "among the worst terrorism we have experienced in the United Kingdom" and that the city had fallen victim to a "callous terrorist attack".
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The target was a pop concert, the audience was a mixture of teenagers, many of them young girls, all out for a fun and innocent evening. Some were young enough to need chaperoning by parents or grandparents.
If this does turn out to be an Islamist-inspired attack, the attacker has deliberately targeted everything his warped beliefs hate in a Western society.
He has also demonstrated a deadly competence - he blew himself up as the high-spirited crowd streamed out of the arena after the concert.
The timing, and location of the explosion - just outside the main arena itself - suggests planning and shows he probably carried out a recce.
The singer Ariana Grande is world-famous. She has more than 45 million followers on Twitter. Another basic but twisted way of guaranteeing this attack will resonate far.
The morning after the attack, a number of things will be happening simultaneously and with urgency.
In Manchester, counterterrorism police from North West Command will be carrying out forensic work at the scene of the explosion.
They will try and find bomb-making signatures that might give a clue as to who was behind this attack.
They will look for certain chemicals, such as triacetone triperoxide (TATP).
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TATP has been used by terrorists around the world, it is a favoured compound of Islamic State and it is relatively straightforward to make, but it is extremely unstable and lethal.
It has been confirmed that the attacker died on the scene - this tells us that something is left of his body.
That will be important in identifying him, either facially, through fingerprints, dental records or DNA.
Once the security services are confident they have the right man, databases will be scoured. Was the attacker known to the security services? Does he have known associates?
His home will be searched. So too the homes of any close relatives or friends.
People might be detained, questioned and then released. Computers will be taken away and their contents and internet history studied.
GCHQ will be looking for a digital trail and if necessary MI6 will speak to foreign partners to build a picture of who this man was and who he knew.
Was he acting alone, a so-called lone-wolf (unlikely with this nature of attack), did he have supporters helping him in the UK, was he being remotely 'directed' by a centralised body (for example IS leadership in Raqqa)?
These are all questions which will have been asked by the Prime Minister as she chaired the COBRA meeting.
The most pressing questions of all though - was this part of a network and should the UK prepare for a secondary attack?
COBRA (the dramatic acronym for the mundane Cabinet Office Briefing Room) is attended by key members of Government, including the home and defence secretaries, the heads of the UK's intelligence agencies and other relevant figures.
The Foreign Secretary is in Brussels so wouldn't have been there in person but could have dialled in, if necessary.
The decision to raise the national threat level is one for JTAC - the Joint Terrorism Analysis Centre.
It sits inside MI5's headquarters on the banks of the River Thames, and acts independent from government although with input from government officials.
The threat level is currently set at "severe", one from the top. It means an attack is highly likely.
If JTAC assesses that another attack is under way or imminent, it will raise that level to the highest - "critical".
The security services have been warning that London isn't the only target of terrorists wishing to attack the UK.
Many smaller cities might be less prepared for an incident like this, but not Manchester.
MI5, which has described the attack on the pop concert as "disgusting", has an office in the North West.
The police armed response unit is highly trained and on the scene within minutes of the attack.
Now and in the coming days, armed police will patrol Manchester city centre, partly for reassurance, partly for increased security.
And then there are 22 families, the morning after, at home, mourning the death of loved ones.
"The death of these children will remain with us forever," a member of the city council told Sky News.
