Detectives are probing a "network" linked to the Manchester suicide bomber Salman Abedi police have confirmed, as his father and brother were detained in Libya.
Twenty-two people were killed in a blast at the Manchester Arena on Monday night, including an eight-year-old girl and an off-duty female police officer, moments after US singer Ariana Grande finished performing, at around 10:30pm.
Prime Minister Theresa May has raised the UK threat level to the highest possible rating, meaning another terrorist atrocity is expected imminently. Almost 1,000 troops have been deployed to key locations.
Here's everything we know about the bomber, the arrests, the victims, and how the government is dealing with the threat.
What is the latest?
Police are investigating associates of Abedi and are carrying out "extensive searches" across Manchester after revealing they believe they're investigating a "network".
Abedi's father Ramadan has reportedly been arrested in Tripoli, Libya. It follows an interview in which he denied his son Salman's involvement in the blast and said: "We don't believe in killing innocents. This is not us."
Abedi's younger brother Hashem has also reportedly been arrested in Tripoli, Libya, on suspicion of having links to the Islamic State group that claimed responsibility for the Manchester Arena attack on Monday.
How many people have been arrested in the UK?
Five people have been arrested in the UK so far.
The first arrest happened on Tuesday in Chorlton, Manchester where a 23-year-old man was held, believed to be Abedi's older brother Ismail. He remains in custody.
Another three men were arrested in south Manchester on Wednesday morning. A fifth suspect, believed to be carrying a package, was arrested in Wigan on Wednesday afternoon.
What is the UK threat level?
Mrs May raised the UK threat level to critical on Tuesday evening, saying a "wider group of individuals" could have been involved in the Manchester Arena blast rather than just Abedi.
What is Operation Temperer?
Operation Temperer is the government plan to put up to 3,800 soldiers on the streets in response to a major terrorist threat. It was devised in 2015 and had been a secret until it was accidentally leaked to a newspaper.
Mrs May said last night that the operation was now in force and armed soldiers would guard "key locations" across London. These include Buckingham Palace, Downing Street, the Palace of Westminster and embassies.
The Palace of Westminster is closed to the public and the Changing the Guard ceremony at Buckingham Palace is cancelled to redeploy police officers.
The move is temporary but it's not known how long the heightened state of alert will remain in place.
The deployment is also unprecedented and puts Mrs May at odds with her predecessor, David Cameron, who was reluctant to use the controversial power.
What do we know about Salman Abedi?
Abedi was born in Manchester in 1994 and is of Libyan descent.
The second youngest of four children, his parents Samia Tabbal, 50, and father, Ramadan Abedi, a security officer, were Libyan refugees who came to the UK to escape the Gaddafi regime, although they are believed to have returned.
Abedi was educated locally in south Manchester and studied business at Salford University – but dropped out before completing his degree.
He is thought to have attended the Manchester Islamic Centre, also known as Didsbury Mosque, along with his parents and siblings.
A family friend, who asked not to be named, described him as "normal" and said the family were known to the Libyan community in the city.
French interior minister Gerard Collomb has said Abedi is believed to have travelled to Syria and had "proven" links with the terror group Islamic State of Iraq and the Levant (Isil).
Mr Collomb told French television that both British and French intelligence services had information that Abedi had been in Syria.
Home Secretary Amber Rudd said Abedi was known to the intelligence services "up to a point".
On Wednesday, BBC News reported 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 broadcaster that two people who knew Abedi at college tipped off officers after he made statements "supporting terrorism" and expressing the view that "being a suicide bomber was OK".
The calls are thought to have been made five years ago, after Abedi left school, the community worker added.
Who are the victims?
Many children and young people are among the dead and some people are still missing.
The first victim to be named on Tuesday was college student Georgina Callander. The 18-year-old "superfan", from Whittle-le-Woods in Lancashire, had met her idol Miss Grande in 2015 and had posted excitedly about the moment on Instagram.
Eight-year-old Saffie Rose Roussos from Leyland, Lancashire is the youngest victim so far. She was caught in the blast after becoming separated from her mother Lisa and sister Ashlee Bromwich, who is in her 20s. They are both being treated for shrapnel injuries.
Olivia Campbell, 15, from Bury, was also killed, her mother said on Facebook on Tuesday night.
Teenage sweethearts Chloe Rutherford, 17, and Liam Curry, 19, from South Shields, were "inseparable" and "beautiful inside and out", their grieving relatives said in a statement released through police.
Tributes have also been paid to 26-year-old John Atkinson from Bury, and Marcin and Angelika Klis, Polish parents of a student at the University of York, who were both at the concert to pick up their children.
Kelly Brewster, 32, from Sheffield was also killed and leaves behind a young daughter, Phoebe. Mothers Alison Howe, 45, and Lisa Lees, 47, from Royton, Oldham, died as they were waiting to collect their teenage daughters.
School receptionist from Blackpool, Jane Tweddle-Taylor, 51, had gone to the Manchester Arena with a friend to pick up the friend's daughter when she died in the blast. The family of another victim, mother-of-three Michelle Kiss, from Lancashire, said she had been taken away in the "most traumatic way imaginable" as they vowed to "draw from the courage and strength she showed in her life".
Sorrell Leczkowski, a 14-year-old from Leeds, was with her mother and grandmother when she died – both of whom are recovering in hospital.
Police have also confirmed that a serving Cheshire Police officer was killed. The BBC reported that the officer was with her husband and two children, who were all injured, the husband critically.
As many as 64 people have been hurt and 20 are being treated for critical injuries. Twelve of those rushed to hospital were children.
Where did the attack happen exactly?
Who can I call if I am concerned about a loved one or have information?
Anyone with concerns over loved ones can contact 0161 856 9400 or 0161 856 9900 for assistance.
The anti-terrorist hotline is 0800 789321. Anyone with urgent concerns should contact 999.
Is the timing relevant?
The blast occurred on the anniversary of the murder of soldier Lee Rigby, who was hacked to death on a London street on May 22, 2013.
Rigby's gruesome murder gained international notoriety when Michael Adebolajo was filmed by passers-by standing in the street with blood-soaked hands trying to justify the attack.
What about the June 8 election?
General Election campaigning will get back under way nationally on Friday after a three-day pause in respect for the victims of the Manchester bomb.
Conservatives, Labour, Greens and the Scottish National Party all announced they will restart low-key local campaigning on Thursday, before resuming the national contest the following day.
Ukip became the first party to say it would be resuming its national campaign. Leader Paul Nuttall unveiled his party's manifesto on Thursday.