What do we know about the suspected attack at Ariana Grande concert

At least 19 people were killed and more than 50 wounded in an explosion at the end 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 19 people 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 blast sparked panic as thousands of people rushed for the exits, witnesses told Reuters.

* U.S. singer Ariana Grande had just finished a concert at the Manchester Arena, the largest indoor arena in Europe that can hold 21,000 people, when the blast occurred.

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.

* Police said they were called at 10:33 pm (2133 GMT) after reports of an explosion.

Manchester Chief Constable Ian Hopkins said police were treating the blast as a terrorist incident and were working with counter-terrorism police and intelligence agencies. They gave no further details on their investigation.

* It is unclear where exactly the blast occurred, but initial reports indicated it happened either just outside the Manchester Arena or near a foyer.

It is also unclear whether it was a bomb. Police have so far not said what caused the blast.

* U.S. officials told Reuters that the timing and venue suggested a terror attack, possibly by a suicide bomber.

"This does not appear to have been a carefully planned attack involving multiple actors, extensive surveillance of the target or exotic materials," said another U.S. official.

"That is what is so worrisome about this kind of thing - how simple it is to indiscriminately kill, wound and terrorise innocent people. With our partners, the U.S. has begun the process of combing through the available intelligence to see if anything was missed."

* 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.

* What about the June 8 election? PM May's ruling Conservative Party, which has a big lead in opinion polls, is preparing to suspend election campaigning due to the blast.

* No militant group has claimed responsibility so far but Islamic State supporters celebrated on social media. Twitter accounts affiliated to the militant Islamist group 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 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.