The attack raised immediate comparisons with the assault on the Bataclan theatre in Paris in November 2015 that left 90 dead.
Police confirmed Tuesday that youngsters were among the 22 victims killed when a suspected lone suicide bomber targeted people leaving a concert by US star Ariana Grande at the Manchester Arena in northern England.
The attack raised immediate comparisons with the assault on the Bataclan theatre in Paris in November 2015 that left 90 dead, with concert-goers to Monday's event wondering why they underwent few security checks.
Witness Chris Pawley told Fox News that "I've been to concerts before and sometimes you get patted down, or have to empty your pockets. There was absolutely nothing at this concert tonight, we literally just got our ticket scanned and were straight in."
Twitter user "Cellie" wrote: "security is not good at Manchester Arena, no one was checking our bags or jackets," although concert-goer James disputed the claims to Sky News, saying "bags were checked security was really, really good."
Manchester Arena stressed the incident had taken place "outside the venue in a public place", suggesting that the attacker had exploited a weak point by targeting the venue's foyer area, which connects to the Victoria train and tram station.
"Several indicators point to the fact that the attack was meticulously planned, and is likely to have involved more than one individual," said Kit Nicholl, security analyst at IHS Markit.
"The positioning of the suicide bomber at the time of detonation, which maximised casualties within an enclosed space while also managing to evade security checks, suggests that a significant amount of planning had gone into the operation," she added.
By waiting for guests to leave through the security ring before detonating, the attacker had illustrated "the vulnerability of such mass gatherings despite security measures in place at the venues themselves," according to Otso Iho from Jane's Terrorism & Insurgency Centre (JTIC).
Event organisers like Live Nation, which runs many venues in London, beefed up security following the Bataclan attack, but did not reveal publicly what measures had been taken.
Security providers are also wary of being too overbearing, according to Simon Battersby, director with the Showsec group which lists Manchester Arena as a client.
He told the Venue Summit in March that "people want to feel confident but also relaxed" and that "it's important customers feel confident as they're coming in, but we need to do it in a friendly way".
If confirmed that the attack took place in a public area, attention will likely shift to the national security services and local police.
The government's Joint Terrorism Analysis Centre (JTAC) provides general guidance by providing sites with a national threat level, which was set at "severe" -- meaning an attack was highly likely.
The guide helps security providers plan their response, but does not force them to implement specific measures such as metal detectors.
The use of an explosive device marks an escalation from recent attacks in Britain, which have used unsophisticated weapons such as knives and vehicles, with firearms less of a threat given the country's strict laws on gun ownership.
It will be of particular concern given that measures are already in place to prevent bomb-making, according to experts.
"It is exceptionally difficult to source the materials necessary to build an improvised explosive device while managing to avoid detection," said Nicholl.
"Since the 2005 London bombings, measures have been put in place to restrict the purchase of materials that can be used to make explosives."
Security services are also likely to come under pressure after Prime Minister Theresa May said police already knew the attacker's identity.
Authorities believe there are around 2,000 potential terrorists in Britain, with the question of surveillance resources sure to come up as the country prepares to go the polls on June 8.