BRITISH spies believe they have identified the suicide bomber who killed 22 people in Manchester, Theresa May revealed today.

Speaking at a press conference outside 10 Downing Street this morning, the Prime Minister said security services are working to establish if he was acting alone or as part of a terror cell.

Her comments came as cowardly ISIS thugs claimed responsibility for the atrocity and cops arrested a 23-year-old man in connection with the attack in a fast-moving probe.

5 Theresa May this morning announced that authorities knew the bomber's identity, but would not yet be publicly announcing it Credit: Getty Images

5 Armed police at Manchester Arena after last night's suicide bombing Credit: PA:Press Association

5 Forensic officers leave Manchester Arena after investigating the scene of an explosion Credit: Getty Images

Mrs May condemned the "cowardice" of the attacker and hailed those who rushed to help, who had shown "the spirit of Britain... a spirit that through years of conflict and terrorism has never been broken and will never be broken".

She vowed: "The terrorists will never win and our values, our country and our way of life will always prevail."

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As the investigation gathered pace, terror experts warned the attack was likely carried out by a cell known to the security services.

It comes as eight-year-old Saffie Rose Roussos was today named as the second person killed in the attack, alongside John Atkinson and Georgina Callander.

Raffaello Pantucci, director of international security studies at Royal United Services Institute, said: "If it is an individual acting alone it seems to be a break from what we have seen in recent attacks.

"But groups have never stopped wanting to launch bombs, it just became more difficult as it requires materials.

"If it is a complete loner then it's a real shock to the system because that means the security agencies have been looking in the wrong places, so I hope they have some prior knowledge of the individual.

"If we are looking at a violent Islamist, it would be surprising security agencies hadn't come across him before."

5 An armed officer guards the scene after the explosion in Manchester overnight Credit: Eamonn and James Clarke

Former MoD counter-terror boss, Major General Chip Chapman, also said the attack bore the hallmarks of a terror network.

He told Sky News: "They'll be looking at whether there's a network there or whether this is a lone individual.

"Most of the time it's not a lone individual, it's very rare that you'll find someone who's able to construct a bomb as a lone individual."

Another expert, former National Counter Terrorism Office member Lee Dodderidge told Radio 5 Live the attack "appeared to have taken quite a considerable amount of planning".

He added: "I probably think this guy was on a database."

Authorities are attempting to piece together what happened outside the Ariana Grande concert overnight.

There are children among the 22 killed, Greater Manchester Police chief constable Ian Hopkins said.

He added that the lone male attacker, who died in the blast, was carrying an improvised explosive device which he detonated.

Shocking pictures and video footage have shown injured gig-goers leaving the 21,000 capacity arena as passers-by rushed to their aid.

Extra police poured on to the streets in the wake of the attack Credit: PA:Press Association

Paramedics at the scene told the BBC of "shrapnel-like injuries", suggesting the explosion could have been caused by a nail bomb.

Security expert Will Geddes stated the attacker had deliberately targeted a security soft spot where concert-goers would be catching public transport home.

He added that the device would have been made from parts easily purchased on the high street but it was unlikely the bomber will have carried out the attack on his own.

Mr Geddes, CEO of security consultants ICP, said: "They are always IED homemade devices, this is a shrapnel based device, shards of metal, could be nuts and bolts, screws which are designed to cause as much damage as possible.

"When a bomb goes off it is the shrapnel from the explosion which has the biggest impact which is often why terrorists use bags of ball bearings inside.

5 A man embraces a woman and a teenager as he collects them from the Park Inn Hotel where they were given refuge after last night's explosion Credit: Getty Images

"He is thought to be a lone suicide bomber and that makes it a lot easier to detonate than leaving a device and detonating it remotely, but I do not subscribe to the lone wolf theory.

"It is very difficult to build these devices on your own. He will have likely had help either in person or electronically through messaging apps or online forums.

"They will have been preparing for at least a month, this is not days, this is weeks, months or even years of preparation.

"No doubt they would have carried out some reconnaissance, it will have been very well planned and the individuals might have regarded the security at the stadium as too difficult and chosen the walkway as the next best option."

However, a former counter-terror officer noted to Buzzfeed that the attacker's assistance was not necessarily based in Britain.

They said: "Don't assume co-conspirators have to be in the UK. The web has rendered physical cells redundant in modern day terrorism."

Anyone with images or footage that can assist the investigation should be uploaded to UK police image appeal online.

Mr Hopkins urged worried families to call the emergency hotline on 0161 8569400 and told people of Manchester to "remain vigilant".