Police scrambled to close down a network around the Manchester suicide bomber with arrests in Britain and Tripoli on Wednesday, as details about the investigation were leaked to US media, infuriating authorities who fear a second attack is imminent.
British-born Salman Abedi, 22, who was known to security services, killed 22 people at a concert venue packed with children on Monday.
Authorities believe he had help in building the bomb, which photographs published by the New York Times showed was sophisticated and powerful, and that his accomplices could be ready to strike again.
Manchester police arrested five men and one woman on Wednesday, bringing the total held for questioning to seven, and searched multiple addresses in northern and central England.
Explosives were found at one site, the Independent reported, citing security service sources.
A source said British investigators were hunting for anyone who may have helped build the suicide bomb.