Two more people arrested earlier in the week have been released without charge.
Greater Manchester police have released images showing suicide bomber Salman Abedi on Monday night, taken shortly before he detonated a bomb at Manchester Arena killing 22 persons.
Twenty-two people were killed in the attack on Monday night, including seven children.
Britain lowered its security threat level to "severe" today following significant activity by police investigating the suicide bomb attack in Manchester, Prime Minister Theresa May said.
Greater Manchester Police said 12 locations had been searched and thousands of items examined.
Police say that 1,000 people are working on the investigation, trying to track down Abedi's accomplices and piece together his movements in the days before he detonated a bomb at an Ariana Grande concert.
"Even with all the progress made so far, there still remain important lines of enquiry that will further build our confidence and understanding of the events that led to this ghastly attack", Assistant Commissioner Rowley said.
At an emergency COBRA meeting, the independent Joint Terrorism Analysis Centre advised that the United Kingdom should return to the second-highest level, which indicates an attack is highly likely rather than imminent.
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Meanwhile, the manager of a pizza shop raided by police in north England claimed that his friend Aimen Elwafi was one of the eight men in custody as he had inadvertently rented a property to Abedi.
He praised the "incredible response" from the public, who he said refused to be divided after such an attack.
He said fans coming to sports matches this weekend should come earlier than usual because of added security screening.
Hundreds of soldiers replaced police at high-profile sites including Buckingham Palace and Parliament, and police armed with submachine guns have been deployed in city centers, transit hubs, tourist areas and major events.
Libyan officials said he and his brother Hashem belonged to the Islamic State group, while their father Ramadan once belonged to a now-disbanded militant group with alleged ties to Al-Qaeda.
El-Khayat said: "If we are radicalizing people, who god-forbidden, have any intention to do (harm), the police will be the first to know".
Ariana Grande has vowed to return to Manchester to hold a benefit concert raising raise funds for the victims of the terror attack at her show in the city.
Northern rail said Friday that Manchester Victoria Station "suffered structural damage that still needs to be properly assessed and repaired".
Armed police were on patrol as the country enjoyed a long holiday weekend with security boosted at Saturday's FA Cup final at London's Wembley Stadium, where Arsenal beat Chelsea.