Seven arrests have now been made in Britain and Libya as police scramble to close down a network around Manchester suicide bomber Salman Abedi.
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.
Abedi's two brothers and father are among those arrested.
- 22 people were killed in the attack at the end of Ariana Grande's Manchester Arena concert on Monday night (local time). At least 64 people were hospitalised.
- UK Prime Minister Theresa May raised the UK's terror threat from 'severe' to 'critical' meaning another attack could be imminent.
- At least 1000 armed troops are out in force around busy London areas.
- Abedi had 'terrorist training' - reports
British-born Abedi, 22, who was known to security services, killed 22 people and injured 64 others 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.
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.
"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."
Abedi, who was born in Manchester in 1994 to Libyan parents, blew himself up on Monday night at the Manchester Arena indoor venue at the end of a concert by US pop singer Ariana Grande attended by thousands of children and teenagers.
Grande was scheduled to perform two shows in London this week but has suspended her tour, her representative said.
Police in Tripoli on Wednesday arrested Abedi's younger brother and his father, who said he did not expect the attack.
"I spoke to [Salman Abedi] about five days ago ... there was nothing wrong, everything was normal," Ramadan Abedi told Reuters, moments before he was arrested.
A spokesman for the local counter-terrorism force said his brother Hashem Abedi was arrested on suspicion of links with Islamic State and was suspected of planning to carry out an attack in the Libyan capital.
The first arrest made in Britain on Tuesday was reported by British and US media to be Abedi's older brother.
Earlier, interior minister Amber Rudd said the bomber had recently returned from Libya. Her French counterpart Gerard Collomb said he had links with Islamic State and had probably visited Syria as well.
British Prime Minster Theresa May will meet US President Donald Trump at a NATO summit in Brussels on Thursday, but officials say she will cut short the second leg of her trip to the G7 summit in Italy. A general election for the UK is also scheduled for June 8.
The 22 victims in Manchester 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.