Salman Abedi, a 22-year old was identified as the suicide bomber who killed 22 people, including children, in an attack on a crowded concert hall in Manchester.
The police said 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 “defenseless 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.
“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, defenseless 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.