The UK's terror threat level is at critical and nine people have been arrested after a man, thought to be a 'mule', killed 22 people at Manchester Arena.
Salman Abedi, a Manchester-born 22-year-old, blew himself up outside the stadium at the end of an Ariana Grande concert held there on Monday, local time.
As well as the 22 people killed, more than 120 were injured.
Seven people have been arrested in the UK since Monday night, including Abedi's older brother, Ismael, who was detained in Chorlton, south Manchester, on Tuesday.
The latest was a man in Nuneaton, Warwickshire, following police searches.
Earlier, a woman was arrested following an armed raid at a block of flats in Blackley in the Greater Manchester area.
Abedi's younger brother, Hashem, and his father, Ramadan, were also arrested in the Libyan capital of Tripoli on suspicion of links with so-called Islamic State, a local counter-terrorism spokesman told Reuters news agency.
Before news of the Tripoli arrest emerged, Greater Manchester Chief Constable Ian Hopkins said on Wednesday he thought it was "very clear that this is a network that we are investigating".
"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."
Home Secretary Amber Rudd said the attack was more sophisticated than some of the attacks seen before.
"And it seems likely - possible - that he wasn't doing this on his own."
BBC security correspondent Frank Gardner said the bomber was thought to have been a "mule", using a device built by someone else.
Police also raided a block of flats near Manchester Piccadilly station in the city centre, requiring them to carry out a controlled explosion and briefly close the railway line.
The terror threat level in the UK was yesterday raised to its highest level, critical, with Prime Minister Theresa May saying a further attack was likely and possibly imminent.
Mrs May said soldiers were being placed at Buckingham Palace, Downing Street, embassies and the Palace of Westminster to support armed police in protecting the public.
A total of 900 soldiers have been deployed across the country.
Buckingham Palace will hold a minute's silence on Thursday to honour the victims, with 14 of them having been named.
Chelsea Football Club have cancelled their Premier League victory parade, due to take place in London on Sunday.
Elsewhere, extra police officers have been deployed at key sites and on transport networks in Northern Ireland and Police Scotland are reviewing security at major events, including the Scottish Cup Final.
Grande has also officially suspended her concert tour including cancelling shows in London and Switzerland.
Bomber had difficulties
Salman Abedi is understood to be a 22-year-old born in Manchester to parents of Libyan descent, and a former University of Salford student.
He attended Burnage Academy for Boys in Manchester between 2009-11.
Hamid El-Sayed, who worked for the UN on tackling radicalisation and who now works at the University of Manchester, said Abedi had a "really bad relationship" with his family and his parents had tried but failed to keep him on the "right path".
"Eventually he was doing very bad at his university, at his education, and he didn't complete, and they tried to take him back to Libya several times. He had difficulties adjusting to European lifestyle," he added.
A former classmate of Abedi's told the BBC that he was a "very jokey lad" but also "very short tempered" and would get angry at "the littlest thing".
The man, who did not want to be identified, said Abedi was "away at random times throughout the year" but he did not know if he was abroad or playing truant because he hung around "the wrong crowd and was very, very gullible".
"You could tell him anything and he would pretty much fall for it."
He said that, before leaving the school in 2011, Abedi became "more and more religious" and that this might explain why he cut ties with former classmates.
A Muslim community worker, who did not want to be identified, has told the BBC that two people, who knew Abedi at college, made separate calls to a hotline to warn the police about his extremist views.
He said they had been worried that Abedi was "supporting terrorism" and had expressed the view that "being a suicide bomber was ok".
The friends had argued with him, telling him he was wrong but had become so concerned they contacted the police, the community worker said.