LONDON — An explosion described by police as a likely terrorist attack ripped through a crowd of teenagers and other concertgoers late Monday after a performance by an American pop singer in the northern England city of Manchester, leaving at least 22 people dead and about 50 others injured.

Initial evidence at the scene suggested the attack may have been a suicide bombing, said two U.S. security officials, who spoke on the condition of anonymity to discuss an ongoing investigation. British authorities, who were holding emergency meetings in Manchester and London, did not immediately confirm those reports.

Armed police gather at Manchester Arena after reports of an explosion at the venue during an Ariana Grande concert. Peter Byrne/PA via Associated Press

The bombing appeared intended to inflict the maximum possible damage on young concertgoers – many of them in their early teens – who were making their way out of the Manchester Arena. Police said the blast occurred about 10:30 p.m., minutes after pop star Ariana Grande had finished her set.

“This is currently being treated as a terrorist incident until police know otherwise,” the Greater Manchester Police said in a written statement.

British Prime Minister Theresa May issued a statement in the early hours of Tuesday saying that authorities were “working to establish the full details of what is being treated by the police as an appalling terrorist attack.”

If confirmed as a terrorist incident, it would be the worst strike on British soil since 2005, when Islamist extremists bombed the London subway and a bus, killing 54 people.

The U.S. Department of Homeland Security said late Monday that there was “no information to indicate a specific credible threat involving music venues in the United States,” but that Americans may see “increased security in and around public places and events as officials take additional precautions.”

Britain has been on high alert for a major attack for several years, with authorities saying that a mass-casualty attack was likely.

Manchester police said they were working closely with national authorities to determine the cause of the explosion. Among the priorities for investigators will be to figure out whether it was part of a broader plot.

Cellphone video showed chaotic scenes of people screaming and running in the aftermath of the blast. The arena was packed with attendees and pink balloons that had fallen from the ceiling during the final song. Initially, concertgoers said they thought popping balloons had set off a panic.

But witnesses later reported seeing the prone bodies of those who had been wounded and killed, as well as others who were streaked with blood and staggering away from the scene. Some were injured in the rush to get out, with people being trampled as thousands rushed to escape.

The singer, who is wildly popular both in Britain and the United States, was “OK,” a spokesman for Grande’s record label told the Reuters news agency.

Concertgoers said they saw nuts and bolts littering the ground near the blast scene and that the smell of explosives hung in the air.

The local hospital, Wythenshawe, said it was dealing with “mass casualties.” Five other hospitals across the city were activated to treat the injured, and emergency supplies of blood were rushed in.

Heavily armed police and emergency services swarmed the arena, with ambulances rushing to the scene.

The local emergency-response service advised the public to call only “for life-threatening emergencies.”

Many of those attending the concert were teenagers venturing to their first concert. Witnesses reported that outside the arena, parents were frantically trying to locate their children. Many parents and teens later gathered at a nearby Holiday Inn that was established as a meeting point.

On Twitter, people offered a place to stay for those stranded in the city using the hashtag #RoomForManchester.

Parents posted pictures of missing children on social media, pleading for information. Police set up a hotline for those looking to connect with missing relatives.

A father told the BBC that he was leaving the arena with his wife and daughter when the blast blew him through a set of doors. Afterward, the man, identified as Andy, said he saw about 30 people “scattered everywhere. Some of them looked dead.”

Separated from his wife and daughter, he said he “looked at some of the bodies trying to find my family.” He later found them, uninjured.

Other witnesses described hearing a loud bang, followed by terrified shouts.

“It was really scary,” Michelle Sullivan, who was attending the concert with her 12- and 15-year-old daughters, told the BBC. “Just as the lights have gone down, we heard a really loud explosion. … Everybody screamed.”

“When we got out, they just said, ‘Keep on running, keep on running.’ ”

Karen Ford, a witness, told the BBC that “there were kids outside, crying on the phone, trying to find their parents.”

About 1:30 a.m., police announced that there would be a controlled explosion after a suspicious object was found. A loud bang was heard minutes later. Police said the item that had been found was discarded clothing, not an explosive device.

The arena is one of the largest indoor venues in Europe, and has a capacity of 21,000. Manchester transport police said the explosion occurred in the arena’s foyer, where people were congregating to buy concert merchandise. Manchester Arena said the attack took place just outside the facility, in a public space.

Although nobody immediately claimed responsibility for Monday’s violence, scenes of bloodied, panicked concertgoers running for safety brought to mind similar images at the Bataclan theater in Paris in November 2015.

The concert hall became the scene of extreme carnage after multiple gunmen burst in during a show by the American rock band Eagles of Death Metal and began shooting. The attack – for which the Islamic State later asserted responsibility – killed 89 people and injured hundreds more, becoming the deadliest event on French soil since World War II.

Monday night’s blast came two months after a speeding driver left four people dead on London’s Westminster Bridge, then stabbed to death a police officer at the gates of Parliament.

Monday was the fourth anniversary of the killing of Lee Rigby, a British soldier who was attacked with a machete on the streets of southeast London. The two assailants, who were convicted of murder, said they were acting to avenge the killing of Muslims by British soldiers.

Monday’s blast comes with just over two weeks to go before Britain holds a national election. It was unclear whether campaigning would continue. Security has not featured as a prominent part of the debate, although that may change.

Grande is a 23-year-old pop singer and actress who has been in the public spotlight since 2010, when she began appearing on the Nickelodeon television show “Victorious.” More recently, she has been touring to promote her third studio album, “Dangerous Woman.” She was scheduled to play two shows in London later this week before traveling to Belgium, according to her tour dates.

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