U.K. raises threat level in wake of concert attack

Despite high-tech security tools and strategy, large public events remain ‘soft’ targets

As officials hunted for accomplices of a suicide bomber and Britain’s prime minister warned another attack could be “imminent,” thousands of people poured into the streets of Manchester in a defiant vigil Tuesday for victims of a blast at a pop concert — the latest apparent target of Islamic extremists seeking to rattle life in the West. The attack left at least 22 dead, including an 8-year-old girl, shattering the revelry at a show by American singer Ariana Grande.

With its long history as a target for terror, England has girded itself in anti-terror technology.

But even with an array of concrete bollards, metal detectors and closed-circuit television cameras with facial recognition to track suspicious people, a terrorist found a weak spot at the 21,000-seat Manchester Arena: the perimeter.

The suicide bombing Monday, which killed at least 22 people and injured 59, demonstrated the difficulty of defending large public sporting and music venues where crowds gather before and after the events. The attack comes despite worldwide strides to improve security since 2001 and the United Kingdom’s efforts since a subway bombing in London in 2005 killed 52 people.

Terrorists know that such venues employ metal detectors, bag checks, sniffer dogs and other security measures at entrances to keep the buildings free of threats, so they look for the soft spots.

The Manchester bombing occurred after the concert in the arena’s foyer, which connects the venue with the Victoria train and tram station.

“Reconnaissance work would likely have been done previously to identify the most vulnerable area at the perimeter of the venue,” said Kit Nicholl, a security analyst at IHS Markit. “The attacker would also have held back for the concert to come to an end in order to target the crowd of people leaving the arena.”

The timing and location of the Manchester bombing suggest the assault was meticulously planned, probably by more than one person, Nicholl said. The Islamic State claimed responsibility for the attack, which U.S. officials were seeking to confirm.

On Tuesday, Prime Minister Theresa May raised the terror threat to the highest possible — critical — and said there are fears that another terrorist at-

KIRSTY WIGGLESWORTH/AP Mourners attend a vigil on Tuesday in Albert Square in Manchester, England.

ANDY RAIN, EPA Mourners remember the people who lost their lives in a terrorist bombing Monday night at the Manchester Arena as Britain remained on alert late Tuesday.