OLI SCARFF/AFP/Getty Images(MANCHESTER, England) — Children are among the 22 killed and 59 injured in a “sickening” attack at an Ariana Grande concert in Manchester, England, Monday night, United Kingdom Prime Minister Theresa May said Tuesday.

Monday’s incident is being treated as a terror attack and May said the threat level remains at “severe,” meaning the government considers another terror attack highly likely. Severe is level 4 out of 5, with critical being the highest.

May said the attacker — who died at the scene after using an improved explosive device — deliberately targeted children and young people “who should have been enjoying one of the most memorable nights of their lives.”

“We struggle to comprehend the warped and twisted mind that sees a room packed with young children not as a scene to cherished but as an opportunity for carnage,” May said.

A suicide bomber was likely behind the attack, officials said, and May said police believe one man carried it out, but authorities still need to know if he was acting alone. May said police believe they know the identity of the suspect but at this stage they cannot confirm his name.

May, who will travel to Manchester Tuesday, said, “there will be difficult days ahead.” She said many people are experiencing unimaginable worry as they still try to find out what happened to their loved ones.

Ian Hopkins, Chief Constable of Greater Manchester Police, said at a press conference Tuesday, “Our priority is to work with the national counter-terrorism policing network to establish more details about the individual who carried out this attack.”

It was unclear if the attacker was acting as a part of a terrorist organization, according to Hopkins, who said the investigation was “complex and wide-ranging.”

“The priority is to establish whether he was acting alone or as part of a network,” Hopkins said. “We would ask people not to speculate on his details or share names.”

President Donald Trump offered his condolences in remarks during a visit to Israel, saying the victims were “murdered by evil losers.”

“I won’t call them monsters because they’d like that term. They’d think that’s a great name,” Trump said in Bethlehem after a visit with Palestinian President Mahmoud Abbas. “I will call them from now on losers because that’s what they are. They’re losers and we’ll have more of them. But they’re losers remember that.”

Greater Manchester Police said they were called to the Manchester Arena just before 10:35 p.m. local time on Monday. The explosion happened inside of the arena’s foyer area after the concert finished, according to witnesses, who reported hearing a loud bang as they exited the arena.

The venue holds about 21,000 people and is one of Europe’s largest indoor arenas, according to its website. The concert arena is connected to the Manchester Victoria Station, the city’s second-largest train station.

Abby Barker, who attended the concert, told ABC News that Grande had just left the stage when there was a loud bang.

“I looked across the arena and everyone over there started running in different directions, screaming in panic,” she said. “We all started panicking too and ran out the doors and ran down the stairs out of the arena. We got outside and children were crying their eyes out, people talking about it being a bomb/gunshots, there were many parents running towards the arena but no one knew exactly what it was.”

Hundreds of police officers were on the scene overnight and transportation security was beefed up in Manchester and in other areas, according to police.

The mayors of several major cities across the globe, including those of London and New York City, also said they were reviewing their security measures as a precaution after the attack.

The British Transport Police department said extra officers would be on patrol at “key railway stations as well as on trains around the country” in the wake of the attack.

“Throughout the day, commuters can expect to see additional officers on their journey,” BTP Assistant Chief Constable Robin Smith said in a statement early Tuesday. “This will include both armed and unarmed officers.”

“As the public would expect in response to an incident such as this, extra BTP officers will be on patrol at key railway stations as well as on trains around the country,” he continued.

An emergency hotline was set up for people concerned for “loved ones who may not have returned home,” police said, and social networking giant Facebook activated its global “safety check” feature in the wake of the deadly attack.

May said General Election campaigning would be suspended.

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