Salman Abedi, 22, has been named by British Police as the suspected suicide bomber who killed 22 people, including children, in an attack on a crowded concert hall in Manchester on Monday night.
Police said they were trying to establish whether he had acted alone or with help from others.
Two U.S. officials who have been in contact with British authorities said he was believed to have traveled to Manchester from London by train.
“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.
Police raided houses in the northern English city and arrested a 23-year old man after Monday evening’s attack, the deadliest in Britain for 12 years.
Prime Minister Theresa May called it an act of “sickening cowardice’’ targeting “defenceless children and young people.’’
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”, but there appeared to be contradictions in its account of the operation.
“One of the soldiers of the Caliphate was able to place an explosive device within a gathering of the Crusaders in the city of Manchester,” the statement said.
“All acts of terrorism are cowardly”, May said in a statement outside her Downing Street office after a meeting with British 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.”
She 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.
The election campaign has been suspended for now as a mark of respect.
May spoke to U.S. President Donald Trump, French President Emmanuel Macron, and several other foreign leaders on Tuesday about the attack, her spokesman said.
She also visited the police headquarters and a children’s hospital in Manchester.
CNN reports that May announced that UK has raised threat level to “critical” — the highest level — for first time in a decade.
Police raided a property in the Manchester district of Fallowfield where they carried out a controlled explosion.
Witnesses in another area, Whalley Range, said armed police had surrounded a newly built apartment block on a usually quiet tree-lined street.
Manchester remained on high alert, with additional armed police drafted in. London Mayor Sadiq Khan said more police had been ordered onto the streets of the British capital.
Monday’s attack was the deadliest in Britain since four British Muslims killed 52 people in suicide bombings on London’s transport system in 2005. But it will have reverberations far beyond British shores.