Kim Hjelmgaard, Jane Onyanga-Omara, and John Bacon

USA TODAY

MANCHESTER, England — A vigil honoring victims of Monday night's terror attack drew thousands of people to a downtown square in this diverse city as the United Kingdom's prime minister raised the terror threat to the highest possible - critical - and said another attack could be "imminent."

In an address focused on the suicide bombing that killed 22 people, including children, British Prime Minister Theresa May said that military personnel could be deployed to help police protect key places around the country and that the police will review how to step up protection at future events. The United Kingdom is "defiant," she said.

"The spirit of Manchester - and the spirit of Britain - is far mightier than the sick plots of depraved terrorists," May said. "That is why the terrorists will never win, and we will prevail."

London Mayor Sadiq Khan told the public in a statement to expect to see more police and more members of the military around the city. "London stands in solidarity with the people of Manchester," Khan said. "We will never be cowed by terrorism."

The hunt for answers intensified Tuesday as the Islamic State claimed responsibility for the attack at an Ariana Grande concert that, in addition to the deaths, left 59 people with injuries.

The explosion rocked the 21,000-seat Manchester Arena on Monday night just as teen pop idol Ariana Grande wrapped up her sold-out show. Chaos ensued as concertgoers, many of them pre-teens and teens, scrambled for the exits amid smoke and screams.

Children as young as 8 years old were among the victims.

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Greater Manchester Chief Constable Ian Hopkins identified the suspected bomber as Salman Abedi, 22, and said he died at the scene. U.S. Director of National Intelligence Dan Coats told a Senate panel Tuesday that ISIS’ involvement had not yet been confirmed.

Hopkins said investigators were working to determine if Abedi acted alone. He said a 23-year-old man was taken into custody after the attack, but did not disclose the man's connection to the investigation. Search warrants were issued at two locations, and explosives were used to gain entry into one of them, Hopkins added.

Hopkins asked for patience and calm in the ethnically diverse city of more than 500,000.

"We understand that feelings are very raw right now and people are bound to be looking for answers," he said. "However, now, more than ever, it is vital that our diverse communities in Greater Manchester stand together and do not tolerate hate."

Thousands turned out for a vigil Tuesday at sun-splashed Albert Square to pay homage to the victims. Poet Tony Walsh drew a huge ovation for reading from his homage to Manchester entitled This is the Place.

"There are hard times again in the streets of our city. We won't take defeat and we don't want your pity," he said. "Because this is the place where we stand strong together with a smile on our face... ​Always remember, never forget, forever Manchester."

As Manchester and the world mourned the victims, online crowdfunding campaigns cropped up to help grieving families pay for funerals and to help the homeless man who rushed inside the arena to help victims. All of the campaigns were launched on the GoFundMe.com website. Among them:

A campaign for the family of 18-year-old Ariana Grande superfan Georgina "Gina" Callander with a goal of 10,000 British pounds or about $13,000 in U.S. dollars. The fundraising page includes a photo of a smiling Gina wearing braces, Ariana Grande's arm around her.

A fundraiser for the family of 28-year-old John Atkinson, described as "a true gentleman" on the GoFundMe page and "a beautiful soul" by a friend on Facebook. The goal is 1,000 British pounds or about $1,300 U.S. dollars.

A GoFundMe campaign for Chris Parker, the 33-year-old homeless man who rushed into the Manchester Arena to help the injured. The goal of the campaign also is $1,000 British pounds or about $1,300 U.S. dollars.

A fundraiser for all of the victims to help with "memorials, funeral costs, bereavement and treatment." The goal of that campaign is 100,000 British pounds or about $130,000 U.S. dollars.

Earlier Tuesday, the ongoing investigation prompted police to urge concertgoers with cellphone video from the blast to come forward. Witnesses shared horrifying tales of the crush of people fleeing the building.

Cherry Nayman, 10, bedecked in glitter and a Beauty and the Beast T-shirt, talked about a stampede as the crowd rushed from the arena.

"My mum was holding onto to us as tight as she could," Cherry said. "On the way out we saw a boy who was probably about 6, and he got pushed over and people were just trampling on him."

Laura Ramsden, from Radcliffe, was one of the last people evacuated from the venue. Ramsden, who was with two friends, has cerebral palsy and watched the concert from the disabled seating area with her electric wheelchair.

"The concert had finished and we were getting ready to leave and then there was a huge bang and a strong burning smell," she said. "People started running and shouting that there was a bomb. People were getting shoved and pushed everywhere."

She said that eventually stewards carried her down the stairs.

"I saw a number of people with blood coming from the head and running down their faces," she said.

Among the victims was Saffie Rose Roussos, 8, from Tarleton, about 40 miles northwest of Manchester. Her teacher at Tarleton Community Primary School described her as a "beautiful little girl in every aspect of the word."

“This has been the most horrific incident we have had to face in Greater Manchester and one that we all hoped we would never see," Hopkins said. He said authorities believe the man detonated an improvised explosive device "causing this atrocity."

A "complex and wide ranging investigation" was underway, Hopkins added.

Queen Elizabeth issued a statement expressing condolences to the victims and "admiration for the way the people of Manchester have responded, with humanity and compassion, to this act of barbarity."

Prime Minister Theresa May traveled to Manchester to meet with police, the mayor and emergency services.

"It is now beyond doubt that the people of Manchester, and of this country, have fallen victim to a callous terrorist attack – an attack that targeted some of the youngest people in our society with cold calculation," May said Tuesday morning outside her London office at 10 Downing Street.

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Pope Francis expressed his "heartfelt solidarity with all those affected by this senseless act of violence."

President Trump condemned the “evil losers” responsible for the attack, saying they preyed on “innocent children.”

The U.S. State Department issued a statement of condolence to victims and their families: "We are working closely with the British government and supporting their efforts to investigate and respond to this attack."

Following the attack, Grande tweeted: “Broken. from the bottom of my heart, i am so so sorry. i don't have words.”

Thousands of people traveled to Manchester, 200 miles northwest of London, for the concert. The city, once a major manufacturing hub, has evolved into a center for film, art and music and is home to two globally recognized soccer teams, Manchester United and Manchester City.

The relatives of dozens of missing concertgoers, many of them children, circulated pictures on Twitter and Facebook in a bid to find them. Later Tuesday, authorities said that all parents and children separated in wild moments after the blast had been reunited.

The Park Inn, one of the hotels directly across from the Manchester Arena, was still filled with families checking out Tuesday, most of them mothers accompanied by one or two young girls.

Tasleem Kamal, from the northern city of Leeds, and her husband were among those who stayed at the Park Inn. The couple hadn't originally planned to attend the show with their daughter and niece but bought tickets at the last minute and went in about halfway through.

"At the end of the show when I heard a big bang I turned to my husband and said, 'Oh, it must be one of the fireworks they sometimes do at the end.' We thought that it was part of Ariana's act," Kamal said.

"But we waited and within 30 seconds we could smell smoke and burning and realized something was wrong. It was just after she finished her final song."

The incident was the worst terror attack in Britain since the suicide bombings on London's transport network in July 2005 that killed 52 people and injured hundreds. The attackers in that blast were inspired by Islamist extremism.

Monday's bombing follows a terror attack near the Houses of Parliament in central London on March 22 that killed five people. The attacker, Khalid Masood, 52, rammed his car into pedestrians on Westminster Bridge before fatally stabbing a police officer. Police believe Masood acted alone and was motivated by Islamist extremism.

All campaigning ahead of the June 8 general election has been suspended.

Manchester has been targeted by terrorism before. The Provisional Irish Republican Army (IRA) detonated a truck bomb in central Manchester's main shopping area in June 1996, destroying the landmark Arndale shopping center and injuring 200 people.

"Although it is not the first time Manchester has suffered in this way, it is the worst attack the city has experienced, and the worst ever to hit the north of England," May said.

Hjelmgaard reported from Manchester; Onyanga-Omara from London; Bacon from McLean, Va. Contributing: Kevin Johnson in McLean, Va., and Melanie Eversley in New York, USA TODAY; The Bolton (England) News