Tragedy in Manchester: Everything We Know About the Terror Attack at Ariana Grande Concert

An explosive went off around 10:33 p.m. local time near the ticket office outside the Manchester Arena, Greater Manchester Police Chief Constable Ian Hopkins said.

Ariana Grande had just finished a concert inside.

Eyewitness Andy Holy told the BBC that he was blown through a door at the arena, noting that the “whole building shook."

Authorities said they suspect the attack was the work of a suicide bomber and were investigating it as terrorism. ISIS later claimed responsibilty, according to one expert group that monitors militants.

Esme Findlay, who was inside the arena at the time, told PEOPLE that "everyone began screaming immediately and pushing to get out" after the explosion.

“No one knew if it was a bomb or what it was, but people were screaming ‘bomb, bomb,’ and there was a lot of panic," she said.

Findlay, 20, told PEOPLE that she heard other concertgoers screaming for their loved ones after the bomb went off.

“Everyone was looking for someone," she recalled. "I saw some girls crying and a woman was asking them where their parents were. It was the scariest thing I’ve ever seen in my life.”

Salman Abedi, 22, is the man police believe detonated the device at the arena, in an apparent suicide bombing.

“The priority remains to establish whether he was acting alone or as part of a network,” Chief Constable Hopkins said at a Tuesday news conference.

Police also arrested a 23-year-old man in connection with the attack, according to Hopkins, and have carried out two local warrants as part of their investigation.

“Broken,” the 23-year-old pop star tweeted late Monday night. “From the bottom of my heart, I am so so sorry. I don’t have words.”

A source close to the situation told Entertainment Weekly that Grande is "absolutely beside herself" as a result of the incident. The source added the star is not focused on the fate of her world tour but is thinking of the victims.

A Grande source told PEOPLE she "is absolutely crushed and devastated as you can imagine. She can’t believe this happened. She just wants to be with her family and loved ones right now."

The attack left 22 people dead, including children, while 59 more were hospitalized and another approximately 60 people were treated as the "walking wounded" but not hospitalized, officials said.

Police have not given many details about the victims, but at least one of the dead is as young as 8 years old.

In the days following the attack, tributes poured in for the 22 victims killed. The youngest fatality, 8-year-old Saffie Rose Roussos, was remembered as "a beautiful little girl in every aspect of the word."

The list of the dead included teenagers, parents, an off-duty police officer and others.

Chief Constable Hopkins gave words of encouragement during a Tuesday news conference:

“Now, more than ever, it is vital that our diverse communities in Greater Manchester stand together and do not tolerate hate.”

A vigil was set for Tuesday at Albert Square in Manchester starting at 6 p.m., according to police.

Insisting another attack was not only likely but possibly imminent, British Prime Minister Theresa May said Tuesday that the country’s terror threat level was rising from “severe” to “critical.”

May made the solemn announcement live on British television as she also cautioned against unnecessary alarm.

She said it is likely the bomber behind Monday’s fatal attack worked in concert with others.

“It is a possibility that we cannot ignore that there is a wider group of individuals linked to this attack,” she explained, adding that soldiers will be deployed to help local police protect city streets nationwide.

The singer was “traumatized” after the terrorist attack at her Manchester concert on Monday, but she is more concerned about the victims, a Grande source tells PEOPLE exclusively.

“Right now, her main headspace and focus is the victims and how she can help. It’s less about her and her own state, and it’s more about them,” the source says of Grande. “She’s alive, she’s safe. This isn’t about her — it’s about the victims.”

Grande landed Tuesday in Boca Raton, Florida, where she reunited with family and her boyfriend Mac Miller. Grande grew up a half-hour away in nearby Coconut Creek, and performed around the Boca Raton and Fort Lauderdale area as a young child, prior to her Nickelodeon career taking off.

Her management also announced her Dangerous Woman tour "has been suspended until we can further assess the situation and pay our proper respects to those lost."

The father and younger brother of the bombing suspect were arrested in Libya, according to multiple reports.

Reuters reports that Ramadan Abedi, the father of suspect Salman Abedi, was detained outside his home in Ayn Zara, a suburb of the Libyan capital city of Tripoli.

Prior to his arrest, Ramadan had said in an interview with the Associated Press that he didn’t believe his son was involved in the attack.

“We don’t believe in killing innocents,” he told the AP. “This is not us.”

The New York Times reports that the suspect’s younger brother, Hashem Abedi, 20, was detained.

The Times reports that in a Facebook post written in Arabic, the Special Deterrence Forces, a Libyan militia, alleged Hashem was a member of the Islamic state who had been involved with the Manchester plot.

The AP reports that neither Ramadan nor Hashem have been charged.

Meanwhile, Manchester police have said that Salman Abedi was part of a larger terrorist network and that he didn’t act alone.

Queen Elizabeth made a surprise visit to a children’s hospital that is treating many of the young victims of the bombing, telling one patient that the attack was “dreadful” and “very wicked.”

The monarch arrived at Royal Manchester Children’s Hospital at around 11:20 a.m. Thursday, shortly after Britain had fallen silent for a minute in tribute to the dead and injured as well as their families and loved ones.

Queen Elizabeth told 14-year-old Evie Mills, who received the concert tickets a birthday present, and her parents, “It’s dreadful. Very wicked. To target that sort of thing,” The Telegraph reports.