Her statement on Tuesday night came after police named the man suspected of carrying out Britain’s deadliest bombing in nearly 12 years as Salman Abedi, 22, but declined to give further details about him.
He was born in Manchester in 1994 to parents of Libyan origin, according to US intelligence officials, and is believed to have travelled by train from London before the attack.
“Our priority, along with the police counter-terrorism network and our security partners, is to continue to establish whether he was acting alone or working as part of a wider network,” Manchester Police Chief Constable Ian Hopkins said.
The attacker set off his improvised bomb as crowds streamed out of the Manchester Arena after a pop concert by Ariana Grande, a US singer especially popular with teenage girls.
“All acts of terrorism are cowardly,” Prime Minister Theresa May said outside her Downing Street office after a meeting with security and intelligence chiefs.
“But this attack stands out for its appalling sickening cowardice, deliberately targeting innocent, defenceless children and young people who should have been enjoying one of the most memorable nights of their lives.”
Britain has increased its security threat level to “critical” from “severe” following the attack, May said in a later televised statement, adding members of the armed forces would boost security at key sites and military personnel might be deployed at public events such as concerts and sports events.
Islamic State, now being driven from territories in Syria and Iraq by Western-backed armed forces, claimed responsibility for what it called a revenge attack against “Crusaders”.
Witnesses related the horror of the blast, which unleashed a stampede just as the concert ended at Europe’s largest indoor arena, full to its 21,000 capacity.
“We ran and people were screaming around us and pushing on the stairs to go outside and people were falling down, girls were crying, and we saw these women being treated by paramedics having open wounds on their legs … it was just chaos,” said Sebastian Diaz, 19.
“It was literally just a minute after it ended, the lights came on and the bomb went off.”
Dozens of parents frantically searched for their children, posting photos and pleading for information on social media.
Singer Grande, 23, said on Twitter she was devastated: “broken. from the bottom of my heart, i am so so sorry. i don’t have words.”
The attack was the deadliest in the UK since four British Muslims killed 52 people in suicide bombings on London’s transport system in 2005.
Attacks in cities including Paris, Nice, Brussels, St Petersburg, Berlin and London have shocked Europeans already anxious over security challenges from mass immigration and pockets of domestic Islamist radicalism.
May said security services were working to see if a wider group was involved in the attack, which fell less than three weeks before a national election.
Campaigning was suspended as a mark of respect.
This pic sums it up what a city #Manchester pic.twitter.com/d49JpiAdVR
— Ben Hulme (@BenHulme_) May 23, 2017
May spoke to US President Donald Trump, French President Emmanuel Macron and several other foreign leaders on Tuesday about the attack, her spokesman said.
Queen Elizabeth held a minute’s silence at a garden party at Buckingham Palace in London.
London Mayor Sadiq Khan said more police had been ordered onto the streets of the British capital.
Police raided a property in the Manchester district of Fallowfield where they carried out a controlled explosion.
On Tuesday evening, thousands of people attended a vigil in central Manchester.
British police do not routinely carry firearms, but London police said extra armed officers would be deployed at this weekend’s soccer cup final at Wembley and rugby at Twickenham.
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