Manchester Attack: Was bomber a 'lone wolf' terrorist?
For the second time in two months, police and security services are embarking on a major investigation into a deadly terror attack.
As with the Westminster atrocity in March, the most pressing question is whether the individual behind the Manchester blast was a so-called "lone wolf" or part of a wider terror cell.
The working theory is that the perpetrator triggered the blast alone but the national police counter-terror network, assisted by MI5, are urgently piecing together his background to see whether he had any help in planning the outrage.
They will be looking to build a picture of the attacker's movements both in recent weeks and months as well as immediately before the strike.
Another priority will be to establish whether any further linked attacks or copycat incidents are planned.
It is likely that the bomber's communications will form a significant part of the inquiry, while investigators will also be checking if he was known to authorities in any way.
One area of focus will be examining the remnants of the device used in the attack as officers work to establish whether the perpetrator built it himself or had help.
As well as seeking to identify any potential accomplices in Britain, authorities will also be looking into the possibility of any link to international groups.
In the first hours after an attack on this scale investigators will be sifting through a number of theories as they work to settle on the most likely lines of inquiry.
The official threat level from international terrorism stands at severe in the UK - indicating that an attack is "highly likely". It has stood at this level - the second highest of five - since August 2014.
There has so far been no indication that this will change in the wake of the events in Manchester.
Shashank Joshi, senior research fellow at security think tank the Royal United Services Institute, said: "The most important point is that police have found the body of what they believe to be the lone suicide attacker.
"If they have identified him, they will be able to begin establishing his movements, his contacts, and his background.
"This, in turn, will help establish whether he acted alone, in concert with a small number of other conspirators, or as part of a larger network.
"The method of attack is likely to downgrade the likelihood that this was perpetrated by a far-right individual or group, as they have not typically used suicide bombers.
"We know that both al Qaida and Islamic State seek to conduct attacks in the UK, and that the UK's terror threat level has been at 'severe' for three years.
"Neither AQ nor IS have formally claimed the attack, and it is too early to speculate further."
Commentators also pointed out that the Manchester attack took place on the fourth anniversary of the killing of Fusilier Lee Rigby in Woolwich, south-east London.
Chris Phillips, the former head of the National Counter Terrorism Security Office, told BBC Radio Four's Today programme: "That may be significant as well."
Belfast Telegraph Digital