Police made four new arrests and searched an address in central Manchester. A source said investigators were hunting for accomplices who may have helped build the suicide bomb and who could be ready to kill again.
"I think it's very clear that this is a network that we are investigating," police chief Ian Hopkins said outside Manchester police headquarters.
"And as I've said, it continues at a pace. There's extensive investigations going on and activity taking place across Greater Manchester as we speak."
Earlier, interior minister Amber Rudd said the bomber, Salman Abedi, had recently returned from Libya. Her French counterpart Gerard Collomb said he had links with Islamic State and had probably visited Syria as well.
Rudd scolded U.S. officials for leaking details about the investigation into the Manchester attack before British authorities were prepared to go public.
The Manchester bombing has raised concern across Europe. Cities including Paris, Nice, Brussels, St Petersburg,
British-born Abedi, 22, blew himself up on Monday night at the Manchester Arena indoor venue at the end of a concert by U.S. pop singer Ariana Grande attended by thousands of children and teenagers.
His 22 victims included an eight-year-old girl, several teenage girls, a 28-year-old man and a Polish couple who had come to collect their daughters.
Britain's official terror threat level was raised to "critical", the highest level, late on Tuesday, meaning an attack was expected imminently.
But, just over two weeks away from a national election, Prime Minister Theresa May's Conservatives and political parties said they would resume campaigning in the coming days.