Just beyond the police cordon surrounding Manchester Arena, residents of the city gathered to lay tributes outside St. Anne’s Church on Tuesday afternoon. The mood was one of “horror and outrage,” said Mark Ashcroft, the bishop of nearby Bolton, after a suicide bomber blew himself up at an Ariana Grande concert on Monday evening, killing 22 and injuring 59 more, including children.

Clare Green, a Manchester native laying flowers at St. Anne’s said, “We do go to the arena a lot for concerts, so it’s a little bit close to home. … Manchester’s been on high alert for quite some time. I’ve always thought security can be better around big events. … It’s not exactly a strict procedure.”

The Manchester attack comes exactly two months after a single attacker drove a car into a crowd of pedestrians in Westminster, killing five people including a police officer and the attacker himself. Until Tuesday night in Manchester, that attack had been the deadliest in the U.K. since the London bombings of July 7, 2005—and the swift and professional response illustrated what security services had learned over the past decade. The 7/7 attack had led to increased investments in security, including specialized training for police officers in responding to terrorist incidents, and improved sharing of intelligence between organizations. And indeed, the measures appeared to have largely worked. As I wrote in March, according to the EU law enforcement agency EUROPOL, nearly half of 211 planned, foiled, or completed terrorist attacks across member states in 2015 were directed at the U.K., which suggested not only that the country was a frequent target, but also that it was uniquely successful at protecting itself. In April, police claimed to have thwarted two terrorist plots.

Salman Ramadan Abedi, who was revealed on Tuesday to have been the Manchester suicide bomber, succeeded anyway. And his plot involved a far more deadly level of sophistication than the use of a simple car and driver. “This is a much more professional-style attack,” said Chris Philips, the former head of the U.K.’s national counterterrorism office, on the BBC.

Manchester’s police chief has now said “this is a network we are investigating;” British Home Secretary Amber Rudd has pointed to the possibility that “he wasn’t doing this on his own.” And “making a bomb that works usually suggests the activity of more than one person,” said Rafaello Pantucci, the director of International Security Studies London’s Royal United Services Institute. “There are likely [to be] people who knew what he was up to at the very least,” he told me. Four people have so far been arrested in connection with the attack.  

Yet something else has changed since the 7/7 attacks, whose perpetrators were unknown to U.K. intelligence services until the attack was carried out. Abedi and the Westminster attacker were each among the roughly 3,000 religious extremists known to MI5, the U.K.’s domestic security and intelligence agency, but neither were thought to pose an imminent threat. This has become something of a pattern in recent terror attacks in Europe. Attackers in Belgium, France, and Germany were also known to security services prior to committing violence, suggesting the difficulty of distinguishing false alarms from real threats.

This also points to the limits of information collection in preventing attacks, and illustrates a broader point about terrorism prevention more generally: The threat is not always apparent until it is realized. Though EUROPOL’s data show the vast majority of plotted attacks are prevented, it also suggests that the U.K. must be inundated with information on possible suspects, and may lack the resources to keep track of them all. It has been reported in the U.K. media that security services have the resources for 24-hour surveillance on less than 50 individuals at any one time.

According to Samir Puri, a security expert at Kings College London, “Counterterrorism work is an art, it is certainly not an algorithm. This means that difficult judgment calls have to be made all the time, not least in terms of how acute the threat is as posed by individuals who may not yet have committed a crime. In this case, an awareness of the possible sympathies or associates of the person in question may not have been enough to accelerate the process of law enforcement intervention.”

As a result of the attack, all campaigning for the U.K. general election on June 8th has ceased for now, but this may not be the only impact for the election. Theresa May’s Conservative government is widely seen to be tough on security, especially in contrast to Jeremy Corbyn, the leader of the opposition Labour Party recently criticized for his past links to IRA terrorists. But voters may equally see the attack as a blight on the Conservatives’ security record. On Tuesday, I asked the U.K. Home Office whether the Manchester bombing could be considered a failure of anti-terror policy. They declined to comment beyond a statement made by Home Secretary Amber Rudd, which did not address that issue.

“Any successful attack is by definition a failure somewhere,” Pantucci told me. “Exactly where this failure lies, in policy, practice [or] intelligence ...  is unclear at this point.”

Pantucci added, “This [attack] shows a level of determination and commitment to cause which is found only amongst a few ideologies. To walk into a crowd of children and blow yourself up like this shows a level of callousness and anger at the world.”