Three men have been arrested in connection with the Manchester concert suicide bombing as a huge investigation into the attack gathers pace.
They were detained after police executed warrants in the south of the city, taking the total of people in custody to four.
Officers were also carrying out a search at an address in Manchester city centre as part of the inquiry into the atrocity.
Twenty-two people were killed and dozens more seriously injured when Salman Abedi detonated a device as fans left Manchester Arena, where US star Ariana Grande was performing on Monday night.
It was the worst terrorist incident to hit the UK since the July 7 attacks in London in 2005.
With Britain on high alert for further attacks as investigators worked to establish whether Abedi was part of a wider cell:
:: France's interior minister disclosed that Abedi, 22, is believed to have travelled to Syria and claimed he had "proven" links with Islamic State;
:: Home Secretary Amber Rudd confirmed the attacker was known to intelligence services "up to a point";
:: 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;
:: Police said they are "confident" they know the identity of all the people who died in the bombing, but they will not be formally named until forensic post-mortems are completed in four to five days.
As the investigation continued, neighbours said a Libyan father, his wife and their two sons, aged in their late teens or early 20s, were taken away by armed police who raided a property on Aston Avenue, Fallowfield, at around 2.30am on Wednesday.
The house is less than a mile from the former home of Abedi.
Neighbour Omar Alfakhuri said at least six police vehicles arrived and armed police took the family out one by one, wearing their bedclothes.
Mr Alfakhuri said the family had lived at the address for 15 years and described the father as religious who prayed at a local mosque.
There was also a raid at a block of flats close to Manchester Piccadilly Station in the city centre.
Prime Minister Theresa May announced late on Tuesday that the terrorism threat level was being raised to "critical" - the highest alert level - meaning that an attack is thought be imminent.
It is the first time the most serious rating has been reached for nearly a decade.
The move reflects fears that a "wider group of individuals" may have been involved in the attack – including the bomb-maker who provided Abedi with his deadly device.
Following Monday's blast, the Government has activated Operation Temperer, providing up to 3,800 troops to support the police in their security operations.
Ms Rudd said the move was temporary but refused to be drawn on how long the heightened state of alert would remain in place.
Following a meeting of the Government's Cobra emergency committee, the Home Secretary said 984 military personnel were being deployed around the country in support of the police.
France's Interior Minister Gerard Collomb told French television that both British and French intelligence services had information that Abedi had been in Syria.
Mr Collomb said: "We only know what the British investigators have told us. He was a British national of Libyan origin, he grew up in Great Britain.
"All of a sudden he travelled to Libya and then most likely to Syria, became radicalised and decided to commit this attack."
When Mr Collomb was asked whether there was a terrorist network, he replied: "We don't know yet (if there's a network). In any case, links with Daesh (Islamic State) are proven."
Ms Rudd confirmed Abedi had recently returned from a visit to Libya, where his parents are reported to now live, 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 told BBC Breakfast.
The Islamic State terror group 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.