British police fear the suicide bomber who killed 22 people at a concert hall in Manchester on Monday may have been backed by a network of accomplices.

Officials say the terrorist cell, if it exists, may want to carry out further attacks as Britain hosts a series of high-profile sporting events in the coming week. On Saturday, Arsenal plays Chelsea in London in the FA Cup Final, and next week, Juventus plays Real Madrid in Cardiff in the final of the UEFA Champions League.

British officials believe the sophistication of the bomb that Salman Abedi detonated at the Ariana Grande pop concert suggests he was not working alone, raising fears that the bomb maker behind him is at large and could send out other suicide bombers.

Ian Hopkins, the chief constable of Greater Manchester Police, said the force was urgently carrying out extensive searches at locations throughout the city.

"It is very clear that this is a network that we investigating, and, as I've said, it continues at a pace," he said. "There's extensive investigations going on and activity taking place across Greater Manchester as we speak."

Police arrested three more people in the city on Wednesday. Ismail Abedi, the brother of suicide bomber Salman Abedi was arrested on Tuesday.

Hashem Abedi, Salman's younger brother was also arrested in Tripoli, Libya on suspicion of having connections to Islamic State, Reuters reported. And the BBC was reporting on Wednesday evening that another arrest had been made, in Wigan, a town within Greater Manchester, bringing the number of UK arrests to five.

Last year, it emerged that Najim Laachraoui, 24, was the bomb maker who provided devices used in attacks in Paris and Brussels. Laachraoui later blew himself up at Brussels airport, possibly because he knew the network had been compromised and that he was at risk of being arrested.

In response to fears that further attacks may be coming in the United Kingdom, the British government raised the nation's terrorism threat level to "critical", which means that another attack could be imminent. Police leave has been cancelled and soldiers have been deployed to patrol civilian areas, both very unusual measures.

Almost 1,000 soldiers have been deployed at Buckingham Palace, the Houses of Parliament, and other sites in London.

Soldiers with special forces equipment were also seen assisting police officers as they raided an apartment in central Manchester.

Ramadan Abedi, the father of Salman, has meanwhile denied that his son was the bomber. The Associated Press contacted him in Tripoli, Libya, where he is the administrative manager of a security force. He told them that his son could not have carried out the bombing and confirmed that another of his sons had been arrested.

"We don't believe in killing innocents. This is not us," he said.

Police, meanwhile, said they believe they have identified all 22 people killed in the bombing. They included two girls of primary school age, a Polish couple who were at the venue to pick up their daughters, and several women who were there to collect children who had been at the concert.

The UK government said on Wednesday evening there would be a minute's silence across the nation at 11 am on Thursday in honor of the victims of the Manchester attack.