Eight people have now been arrested over the Manchester Arena blast that killed 22 people.
Police said they arrested two men on Thursday in the Manchester area in connection with the deadly bombing of Ariana Grande's concert, while a detained woman was released without charges.
British soldiers fanned out to guard key sites as investigators tried to piece together the last movements of bomber Salman Abedi, a Manchester-born university dropout who died in Monday's explosion.
Images published by The New York Times newspaper showed a detonator that he was said to have carried in his left hand, shrapnel including nuts and screws, and the shredded remains of a blue backpack.
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Abedi's bomb killed 22 people, one of whom was just eight-years-old, at a concert by US pop star Ariana Grande.
The device appeared to be fairly sophisticated, according to the photographs that appeared to have been taken at the scene.
A French minister said Abedi may have been radicalised in Syria. His parents had reportedly fled the now fallen regime of Libyan dictator Moamer Kadhafi.
His father and younger brother, Ramadan and Hashem Abedi, were detained in Libya, authorities there said, after the father reportedly insisted on Abedi's innocence.
Wednesday, May 24 photo of Hashem Abedi, brother of Salman Abedi, appearing inside the Tripoli-based Special Deterrent anti-terrorism force unit after his arrest (AAP) Source: AAP
A spokesman for the Deterrence Force, which acts as Government of National Accord's police, said the brother was aware of Abedi's plan and the two brothers were both members of the Islamic State group.
The attack was claimed by IS jihadists.
Hashem had been "under surveillance for a month and a half" and "investigation teams supplied intelligence that he was planning a terrorist attack in the capital Tripoli", the Deterrence Force said on its Facebook page.
A relative told AFP that Abedi had travelled to Manchester from Libya four days before the bombing.
"It's very clear that this is a network that we are investigating," Manchester police chief Ian Hopkins told reporters, with five people now under arrest.
The British government announced a nationwide minute's silence for 11:00am (1000 GMT) Thursday in memory of those killed and the dozens wounded.
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'Act of cowardice'
The attack was the latest in a series of deadly incidents across Europe claimed by IS that have coincided with an offensive on the group's redoubts in Syria and Iraq carried out by US, British and other Western forces.
Officials said Abedi, 22, had been on the radar of the intelligence community before the massacre and warned another attack "may be imminent".
After arresting a 23-year-old man on Tuesday, police said they had taken three more men into custody on Wednesday in south Manchester, where Abedi lived.
A fifth man who was carrying a suspect package was then detained in Wigan, northwest of the city.
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A sixth person, a woman, was arrested by police in Blackley, an area north of Manchester city centre.
The seventh arrest was made following searches at an address in the town of Nuneaton in central England, and is the first outside the Manchester area.
"This evening we have been carrying out searches at an address in Nuneaton, Warwickshire, and arrested a man," the Greater Manchester Police force said in a statement.
"These searches are connected to Monday's attack on the Manchester Arena, but this is a fast-moving investigation and we are keeping an open mind at this stage.
"As it stands, six men and one woman have been arrested in conjunction with the investigation and remain in custody for questioning."
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Elders at the south Manchester mosque believed to have been frequented by Abedi insisted that his actions were wholly alien to their preaching, and pointed the finger at online radicalisation.
"This act of cowardice has no place in our religion," said Fawzi Haffar, a trustee at the Didsbury mosque, after he and other mosque leaders held their own minute's silence.
French Interior Minister Gerard Collomb said Abedi had "likely" been to Syria after a trip to Libya, citing information provided by British intelligence services to their counterparts in Paris.
"In any case, the links with Daesh are proven," he said, using a term for IS.
British Prime Minister Theresa May announced that the country's terror threat assessment had been hiked to "critical", the highest of five levels.
Hundreds of armed military personnel fanned out to take up guard duties at the British parliament and Buckingham Palace -- a highly unusual sight on the British mainland.
The attack was the deadliest in Britain since July 7, 2005 when four suicide bombers inspired by Al-Qaeda attacked London's transport system during rush hour, killing 52 people.
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NATO needs to 'step up'
In light of the Manchester attack, NATO chief Jens Stoltenberg said the military alliance had to "step up and agree to do more in the fight against terrorism" at summit talks set for Thursday.
The summit is to be attended by US President Donald Trump, who has lambasted NATO for not doing more against Islamist extremism.
NATO leaders are expected to formally sign up to the US-led coalition against IS, according to one diplomatic source.
The bombing came just over two weeks before a snap general election set for June 8.
May's governing Conservatives and the Labour main opposition agreed to resume local campaigns on Thursday and national campaigning on Friday.
Football is integral to Manchester culture and a minute's silence was held before the Europa League final in Stockholm between Manchester United and Dutch giants Ajax.
Players stood with their heads bowed, with United sporting black armbands.
Outside the silence, the team's fans chanted "Manchester, Manchester, Manchester!" and unfurled a long red banner that read "Manchester -- a city united".
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'Sewn, bolted, drilled'
Grande's tour was suspended "until we can further access the situation and pay our proper respects to those lost", her management said.
She cancelled her two shows in London scheduled for Friday and Saturday, plus five European dates ahead of her June 7 concert in Paris.
A total of 64 people are being treated in hospital, including 20 in critical care, medical officials said.
Twelve of the injured are aged under 16.
Nick Lewis said his daughter Freya had been through 10 and a half hours of surgery.
"Freya has been sewn, bolted, drilled and bandaged back together. It is going to be a long climb but we are on the first step," he said.
