What happened?Twenty-two people were killed and 59 people were injured in a blast in Manchester on Monday night, police said.
Greater Manchester Police chief constable Ian Hopkins said that the lone male attacker, who died in the blast, was carrying an improvised explosive device which he detonated.
Many of the fans at the concert were young people. The blast sparked panic as thousands of people rushed for the exits.
Grande, 23, later said on Twitter: "broken. from the bottom of my heart, [I] am so so sorry. [I] don't have words."
When and where did it happen?Police said they were called at 10:33pm after reports of an explosion.
It is unclear where exactly the blast occurred, but initial reports indicated it happened either in the foyer of the venue, Europe's largest indoor arena which can hold 21,000 people, or just outside of it.
Is this terrorism?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.
US officials told news agency Reuters that the timing and venue suggested a terror attack, possibly by a suicide bomber.
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.
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.
If it is confirmed as a terror attack, it would be the worst to hit Britain since the July 7, 2005 suicide bomb attacks in central London in which 52 people were killed.
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's the situation now?Police advised the public to avoid the area around the Manchester Arena, and the train station near the arena, Victoria Station, was evacuated and all trains cancelled.
The British Transport Police said that officers will be on patrol at key railway stations as well as on trains around the country.
"This will include both armed and unarmed officers," the BTP said in a statement.
What about the June 8 election?Labour Party leader Jeremy Corbyn says he and Mrs May have agreed to suspend election campaigning until further notice.
Mr Corbyn said on Tuesday he is "horrified" by the events in Manchester and that his thoughts are with the families and friends of those who have died and been injured.
Mr Corbyn says he had spoken with May after the explosion.
Ukip leader Paul Nuttall announced on Tuesday morning that his party would also suspend its campaign.
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