British police said they arrested two more people during raids on Saturday in connection with the suicide bombing that killed 22 people at a Manchester pop concert.
The arrests of the two men, aged 20 and 22, bring to 11 the number of suspects held in Britain over Monday's attack, for which the Islamic State group has claimed responsibility.
Police in Libya have also detained the father and brother of 22-year-old bomber Salman Abedi.
Mark Rowley, head of Britain's counter-terrorism police, said on Friday that police had captured "a large part of the network" linked to the atrocity in which seven children under 18 years old were among the 22 dead.
"We are very happy we've got our hands around some of the key players that we are concerned about but there's still a little bit more to do," he said.
The bombing at a concert by US pop idol Ariana Grande was the latest in a series of IS-claimed attacks in Europe that have coincided with an offensive on the jihadist group in Syria and Iraq by US, British and other Western forces.
Britain's terror threat assessment has been increased to "critical", the highest level, meaning an attack is considered imminent.