MANCHESTER, England (Reuters) - At least 22 people, including children, were killed and 59 wounded by a suicide bomber as thousands of fans streamed out of a concert by U.S. singer Ariana Grande in the English city of Manchester on Monday.
Following is a Reuters summary of what we know and do not know about the incident.
* Death toll: British police said 22 people, including children, were killed and 59 people had been treated in hospital. A total of 60 ambulances attended the incident.
Many of the fans at the concert were young people. The explosion sparked panic as thousands of people rushed for the exits, witnesses told Reuters.
* Police said they were called at 10:33 p.m. (2133 GMT) just after a man detonated explosives among fans.
"We believe, at this stage, the attack last night was conducted by one man," Manchester Chief Constable Ian Hopkins said. "The priority is to establish whether he was acting alone or as part of a network.
"The attacker, I can confirm, died at the arena. We believe the attacker was carrying an improvised explosive device which he detonated causing this atrocity."
More than 400 officers were involved in the operation overnight. Police appealed for the public to upload images and footage to assist them in their investigation.
* U.S. singer Ariana Grande had just finished the concert at the Manchester Arena, the largest indoor arena in Europe with capacity for 21,000 people, when the bomber set off his device.
Grande, 23, later said on Twitter: "broken. from the bottom of my heart, i am so so sorry. i don't have words."
* Parents hunted for missing children after the blast. Many turned to social media to seek loved ones.
"Everyone pls share this, my little sister Emma was at the Ari concert tonight in #Manchester and she isn't answering her phone, pls help me," said one message posted alongside a picture of a blonde-haired girl with flowers in her hair.
* Transport police said they believed the attack had taken place just outside the Manchester Arena near a public foyer which linked to the train station.
* Prime Minister Theresa May said authorities were working to establish the full details of what police were treating as "an appalling terrorist attack". She said her thoughts were with the victims and the families of those who have been affected.
She will hold a meeting of the government's emergency response committee.
* What about the June 8 election? Major British parties have all suspended campaigning.
* No militant group has claimed responsibility so far but Islamic State supporters celebrated on social media.
Twitter accounts affiliated to the jihadists have used hashtags referring to the blast to post celebratory messages, with some users encouraging similar attacks elsewhere.
* 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 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.
Additional reporting by David Milliken, Alistair Smout, Kate Holton and Michael Holden in London, Mark Hosenball and John Walcott in Washington and Mostafa Hashem and Ahmed Aboulenein in Cairo; Writing by Guy Faulconbridge; Editing by Louise Ireland