UPDATED Tuesday, 4:04 PM PT with terror threat level raised: Police have named the suicide bomber behind the Manchester terror attack as 22-year-old Salman Abedi. Greater Manchester police chief constable Ian Hopkins revealed his identity in a media briefing outside the force headquarters on Tuesday afternoon.

Meanwhile, ISIS has claimed responsibility for the attack, though officials have not confirmed that. Later on Tuesday, UK Prime Minister Theresa May raised the nation’s threat level to “critical,” its highest level, as investigators can not determine whether Abedi acted alone. The threat level will result in greater armed military and armed police presence at key sites in the country.

According to a BBC report, Abedi was born in Manchester and has “at least three brothers and sisters.” His family are believed to be Libyans originally, and they lived at more than one address in Manchester.

“We understand that feelings are very raw right now and people are bound to be looking for answers,” said Hopkins in a press briefing. “However, now, more than ever, it is vital that our diverse communities in Greater Manchester stand together and do not tolerate hate. We have been visited by the Prime Minister and Home Secretary and we have taken them through the emergency response so far and what we plan to do in future days.

“I can confirm that the man suspected of carrying out last night’s atrocity has been named as 22-year-old Salman Abedi. However, he has not yet been formally identified and I wouldn’t wish, therefore, to comment further. The priority remains to establish whether he was acting alone or as part of a network.”

PREVIOUS TUESDAY AM UPDATE: British police have arrested a 23-year-old man in connection with the suicide bombing at an Ariana Grande concert in Manchester last night. At least 22 people, including children, are now dead  with a further 59 people injured. Twelve of those taken to the hospital were aged under 16. With flags at half mast at 10 Downing Street, British Prime Minister Theresa May has expressed her sympathies with the families affected in “what is being treated by the police as an appalling terrorist attack.” In messages posted online, Islamic State has claimed responsibility for the attack.

PREVIOUS MONDAY PM UPDATE with news conference: Greater Manchester Police say the biggest attack in the UK since the 7/7 bombings has left 19 people are dead and 59 injured after an explosion outside the Manchester Arena just as an Ariana Grande concert was letting out. The bomb exploded in the foyer outside the arena near a ticket office as crowds were leaving the building.

TV news outlets are quoting multiple U.S. officials who were briefed on the incident as saying British authorities suspect that the attack was carried out by a suicide bomber with a backpack bomb. They added that they have identified the suspect, but no name was released.

“There was a massive explosion, and everyone was screaming,” one witness told Reuters from one of England’s largest cities. “It was a huge explosion; you could feel it in your chest.”

The pop singer, who was not hurt in the attack, tweeted:

The venue is among the largest indoor arenas in Europe, with a capacity of 21,000. Manchester Victoria train station, which is right underneath the arena, has been closed since the bombing.

“We are currently treating this as terrorist incident until we have further information,” Ian Hopkins, chief constable of the Greater Manchester Police, said in a brief statement to reporters. He said his officers are working closely with the national counterterrorism policing network and UK intelligence partners on the incident, which happened at about 10:33 PM local time.

“This is clearly a very concerning time for everyone,” Hopkins said. “We’re doing all that we can, working with local and national agencies to support those affected as we gather information about what happened last night. As you’ll understand, we are still information and updated, so we’ll provide further details when we have a clearer picture.”

He said the injured are being treated at six hospitals in England’s second-largest city. Read his full statement below. Here is some video from inside the hall:

All three U.S. cable news networks pivoted to coverage of the incident. One witness told CNN she saw 20-30 bodies of people “clearly” dead and that there was “blood everywhere.” Pop singer Grande’s reps have told media outlets that the singer is OK.

Video on social media shows people scrambling to exit the venue, and footage from the scene shows a huge police presence and streams of ambulances and other emergency vehicles. Audio from dashcam video taken from a car parked near the arena appears to show one large explosion, possibly followed quickly by a second smaller blast.

“Officers are at Manchester Arena following reports of an explosion within the foyer area of the stadium at 10.30PM this evening,” British Transport Police said in a statement. “Emergency services are at the scene and we are working to establish more information regarding the explosion and will provide further updates as soon as possible.”

The incident comes exactly two months after a terrorist attack in London in which several people died and many others were injured. It also is the four-year anniversary of the brutal hacking death of an off-duty British Army soldier in a London street.