British police moved quickly, arresting a 23-year-old man in connection with Monday night's bombing, carried out as crowds began leaving a concert given by Ariana Grande, a USA singer who attracts a large number of young and teenage fans.
An 8-year-old girl was among the dead - the youngest known victim - and the child's mother and sister were among 59 people wounded in what British Prime Minister Theresa May called "a callous terrorist attack".
May chaired a meeting of her security Cabinet and concluded that 22-year-old British citizen Salman Abedi, who reportedly carried out Monday night's attack, may have been part of a wider network that is poised to strike again.
"We struggle to comprehend the warped and twisted mind that sees a room packed with young children not as a scene to cherish but as an opportunity for carnage", May said Tuesday. Hours after the blast, the Arndale shopping center in South Manchester was evacuated and a 23-year-old was arrested.
"Her team is continuing to assess the situation before they decide how to move forward, " the source says, adding, "their focus is on the victims and their families".
President Trump spoke out about the bombings during his trip to Israel, saying in a statement he would call the terrorists "losers".
"I wouldn't wish to therefore comment any further about him at this stage", he said. About 50 people have been injured.
There was no immediate claim of responsibility, but US officials drew parallels to the coordinated attacks in November 2015 by militants on the Bataclan concert hall and other sites in Paris, which claimed about 130 lives.
Chris Upton, her primary school headteacher, described Saffie as "a attractive little girl in every sense of the word".
Images from the scene show people running in a panic down stairs to escape the 21,000-seat arena amid the relentless screams of frightened children and young teenagers. The bombing took place at the end of the concert, when the audience was streaming toward the exits of Manchester Arena, one of the largest indoor concert venues in the world.
The northern English city remained on high alert, with additional armed police drafted in.
"We had no idea what it was...there was all kinds of speculation as to what was going on. And if it was even possible the spirit of the people of Manchester will grow even stronger this evening", he said as he fought back tears.
Police, however, have spoken only of "an improvised device" used in the attack.
It's not clear whether the group directed the attacker, as it did with the assailants who killed 130 people across Paris in a series of attacks in November 2015.
May condemned the attack for its "appalling sickening cowardice" and for "deliberately targeting innocent defenceless children and young people". "I wrapped her in one of the merchandise T-shirts and I said "where is your mum and daddy?' She said 'my dad is at work, my mum is up there".
"My heart is heavy today as I extend my prayers to the children and families affected by last night's frightful tragedy in Manchester", the artist says in a statement.
"There is always that fear. but I allow her to go because I say we can not allow terrorism to take over our lives", the mother said Tuesday following the Ariana Grande tragedy in Manchester, England.