Manchester Terror Attack: What We Know

Britain has raised its terror alert to the maximum level and ordered troops to protect strategic sites after 22 people were killed in a suicide bomb attack on a Manchester pop concert.

Here is what we know so far about Monday's attack, the deadliest in Britain since 2005.

- What happened? -

Police said they were called at 10:33 pm (2133 GMT) Monday after an explosion at Manchester Arena during a concert by U.S. pop star Ariana Grande, who is popular with teenagers and pre-teens.

Witnesses described a "huge bomb-like bang" and scenes of panic as young fans rushed out and parents waiting outside searched frantically for their children.

The bomber used an improvised explosive device apparently packed with nails and other metal objects outside one of the exits at the 21,000-capacity arena.

Photographs published by The New York Times on Wednesday suggest the suicide bomber -- or a possible team helping him -- had a remote detonator setup, to back up a hand-held detonator found at the scene.

- Who is behind it? -

The suspected bomber has been identified as 22-year-old Salman Abedi, a British student dropout born to Libyan parents who fled the regime of slain dictator Moammar Gadhafi.

Interior minister Amber Rudd confirmed on Wednesday that Abedi was known to intelligence services.

Manchester Chief Constable Ian Hopkins said Abedi had not acted alone, adding it was "very clear that this is a network that we are investigating."

A total of 10 people are being held in Britain and Libya over the bombing since Tuesday, including Abedi's father and brother in Libya. 

Libyan authorities said Abedi's brother had been aware of the attack plan. 

Abedi's father was once part of a Libyan militant group with alleged ties to al-Qaida, a Libyan security source said Thursday.

Eight men remain in custody in Britain, most arrested in and around Manchester, including three men near the house where Abedi lived.

A 23-year-old man arrested Tuesday is likely to be Abedi's older brother Ismael.

A woman arrested Wednesday was released a few hours later without charge.

French Interior Minister Gerard Collomb said Abedi had become radicalized after a trip to Libya and probably Syria as well, according to information received from British intelligence services.

The Islamic State group has claimed responsibility through its social media channels, saying "one of the caliphate's soldiers placed bombs among the crowds," and threatening more attacks.

- Who are the victims? -

Twenty-two people were killed and 116 injured, 75 of whom remain in hospital.

Twenty-three of the injured remain in critical care.

Eight-year-old Saffie Rose Roussis is so far the youngest named victim. She attended the concert with her mother and older sister, who were both injured.

Olivia Campbell, 15, was confirmed dead on Wednesday by her mother, who had issued heartrending appeals for help when her daughter was still listed as missing.

A Polish couple living in Britain, identified as Angelika and Marcin Klis, were also caught in the explosion as they went to collect their daughters.

And police chief Hopkins confirmed Wednesday that a serving police officer was among the victims.

- Security measures -

Prime Minister Theresa May on Tuesday placed the country on its highest level of terror alert -- "critical" -- meaning a new attack is believed to be imminent.

Soldiers have been sent to assist armed police to protect strategic sites. The last time troops were deployed on British streets was in 2007.

The troop plan, codenamed Operation Temperer, was first revealed after the November 2015 Paris attacks and is believed to allow up to 5,000 soldiers to be deployed.

Major sports venues plan to beef up security, with several high-profile events in the coming weeks, including the FA Cup final at Wembley on Saturday.

The Changing of the Guard outside Buckingham Palace was canceled on Wednesday and parliament suspended all public events.

Manchester United's Europa League final against Ajax in Stockholm went ahead Wednesday under tight security and the team dedicated their victory to the victims.

May traveled to Brussels for Thursday's NATO summit but has cut short her trip to a G7 meeting in Italy this week, attending on Friday but returning home before the summit ends on Saturday.

- Other attacks on UK soil -

The Manchester bombing is Britain's second terror attack in two months. 

On March 22, five people were killed and more than 50 injured when a man plowed into pedestrians on Westminster Bridge in central London.

The attacker, 52-year-old Khalid Masood, fatally stabbed a police officer before being shot dead by police outside parliament.

The deadliest bomb attack on British soil took place in July 2005 when four British suicide bombers inspired by al-Qaida attacked London's transport system, killing 52 people and wounding 700.