The Manchester suicide bomber's "network" is the focus of the huge counter-terrorism investigation into the atrocity as members of his family were detained in Libya.
Salman Abedi's father, Ramadan Abedi, has been arrested in Tripoli, along with his brother Hashim, who Libyan security forces said was "aware of all the details" of the attack.
Ramadan Abedi had earlier claimed his son was innocent, saying: "We don't believe in killing innocents. This is not us."
The developments came as the probe was hit by further leaks to the US media, with the New York Times releasing crime scene pictures appearing to show bomb fragments and the hiking backpack used to conceal the explosive.
In other developments on Wednesday, UK detectives made a further five arrests, taking the number of people in custody to six.
With Britain on critical alert for further attacks:
:: Three men were arrested after police executed warrants in south Manchester overnight, while officers entered an address in the city centre using a controlled explosion on Wednesday afternoon;
:: Two further arrests were made later, of a man carrying a suspect package in Wigan and of a woman following an armed raid at a block of flats in Blackley, north Manchester.
:: France's interior minister disclosed that the bomber is believed to have travelled to Syria and claimed he had "proven" links with Islamic State (IS);
:: Almost 1,000 military personnel were being deployed around the country, including to key sites such as Parliament, Buckingham Palace and Downing Street, after the official terror threat assessment was raised to critical, the highest level, indicating that a further attack may be imminent;
:: Claims emerged in America, reported by NBC News, that members of the bomber's family warned security officials Abedi was "dangerous" in the past;
:: Management for US pop star Ariana Grande, whose concert had just finished when the bomb went off, said her upcoming gigs at the O2 in London had been cancelled;
:: The Government announced that a minute's silence will be held at 11am on Thursday in remembrance of those who lost their lives or were affected by the attack;
Twenty-two people were killed and dozens more seriously injured when Abedi, 22, detonated a device as fans left Manchester Arena on Monday night.
It was the deadliest terrorist incident to hit the UK since the July 7 attacks in London in 2005 and sparked a nationwide security operation amid fears further strikes could be imminent.
On Wednesday afternoon, Greater Manchester Police Chief Constable Ian Hopkins said the level of activity in the probe is "intense" and continuing "at pace".
Asked if officers are looking for the person who made the bomb, he said: "I think it's very clear that this is a network that we are investigating and as I've said, it continues at pace, this extensive investigation is going on and activity taking place across Greater Manchester as we speak."
Officers continued to question a 23-year-old arrested in south Manchester on Tuesday.
The force said it was "confident" it had identified every victim but said they would not be formally identified until after post-mortem examinations were completed in four to five days.
Cheshire Police confirmed that one of its female officers died while off-duty at the concert but have not named her.
The BBC reported that her husband is critically ill and her two children injured.
There were reports that members of the public blew the whistle on Abedi several years ago by reporting him to the anti-terrorism hotline.
An unnamed Muslim community worker told the BBC two people who knew the attacker at college tipped off officers after he made statements "supporting terrorism" and expressing the view that "being a suicide bomber was okay".
The calls are thought to have been made five years ago after Abedi left school, the community worker added.
Prime Minister Theresa May will chair a meeting of the Government's Cobra emergency committee in Whitehall on Thursday morning, said Downing Street.
The meeting, bringing together senior ministers, officials, police and security agencies, will be the fourth Cobra gathering to deal with the Manchester bombing.
In response to the heightened threat, the Government has activated Operation Temperer, providing up to 3,800 troops to support the police in their security operations.
Home Secretary Amber Rudd said 984 military personnel were being deployed around the country, which the Metropolitan Police said had freed up 1,000 armed officers to carry out patrols.
France's interior minister Gerard Collomb told French television that both British and French intelligence services had information that the attacker had been in Syria.
Mr Collomb said: "All of a sudden he travelled to Libya and then most likely to Syria, became radicalised and decided to commit this attack."
Ms Rudd confirmed Abedi had recently returned from a visit to Libya, and said the nature of the attack suggested he may have had support.
"It was more sophisticated than some of the horrific events that we have seen in the past or in other parts of Europe so people are reasonably wondering whether he did this on his own," she said.
IS claimed responsibility for the barbaric attack, which involved a home-made device packed with nuts and bolts which exploded in the venue's foyer as thousands of young people were leaving.
A spokesman for the Libyan authorities told BBC2's Newsnight that one of Abedi's final acts before the murders was to ring his mother.
The spokesman said: "His brother felt there was something going on there in Manchester and he thought his brother would do something like bombing or attack. So after that, he told us, 'Having internet, I see the attack in Manchester and I knew that's my brother'."
The spokesman added that Libyan authorities were aware of Abedi going to the capital of Tripoli on April 18 and believed he stayed for two or three weeks.
He revealed that Abedi's younger brother Hashim had been investigated for about a month and a half over suspicions that he was linked to IS.
"We were not quite sure about this, but when we arrested and we asked him, he told us, 'I have ideology with my brother'. Hashim told us, 'I know everything about my brother, what he was doing there in Manchester'."