For more than two years, Britain had been on high alert. As terrorist attacks hit other European countries, UK authorities foiled plots, stepped up security preparations and issued warnings about the threat.
Then, a year ago on Thursday, the danger that had seemed close but not quite here crystallised in 82 seconds in Westminster.
Gunshots and sirens rang out around Parliament after Khalid Masood launched a murderous rampage at the centre of British democracy, killing five people before he was shot dead by police.
The atrocity was a horrific event in its own right, but it was only the beginning of a wave of terrorist activity.
Dozens more would be killed or injured over the next six months as a further four attacks occurred in London and Manchester.
The flurry of incidents prompted a period of intense introspection and pressure for Britain's counter-terrorism agencies.
Hundreds of officers were assigned to five huge inquiries, while questions were inevitably asked about how the attackers had "got through".
In some cases, it emerged, the perpetrators were already on the radar.
An official review found it was "conceivable" the Manchester bombing in May might have been averted "had the cards fallen differently", while the ringleader of the London Bridge outrage in June was under active investigation at the time of the attack.
Masood, 52, was known to security services for association with extremists but he was a "closed" subject of interest when he struck in Westminster.
As well as reviewing past intelligence and records to, as the head of MI5 has put it, "squeeze out every last drop of learning", Britain's counter-terror apparatus is working at an unprecedented pace to fend off future atrocities.
While the attacks that occurred in 2017 provided the most stark indication of what senior officers describe as a "momentum shift", further evidence can be seen in the scale of preventative activity.
In nearly four years before Westminster, 13 attack plots were stopped. In the last year alone, 10 Islamist and four extreme right-wing plots have been foiled.
Terror-related arrests are being made at a rate of more than one a day. But even these figures paint only a partial picture.
At any one time police and MI5 are running more than 600 live counter-terror investigations relating to 3,000 individuals. In addition, officers must also keep an eye on any risk from a growing pool of more than 20,000 people who previously featured in inquiries.
Last year's events demonstrated some of the features that have most alarmed security chiefs:
-The speed of radicalisation, with plots escalating to the point of violence in a matter of days or weeks.
-The ability of extremists to exploit technology, with all five attacks having an "online component".
-A new breadth to terrorist methods, from "low-tech" weapons such as knives and vehicles to more complex bomb attacks.
The shift in tempo has prompted an examination of the powers and tools authorities can deploy to counter the threat.
Measures already announced include: a £50 million boost to the annual counter-terrorism policing budget; an increased role for MI5 in tackling the growing threat from extreme right-wing terrorism; an improved strategy for exploiting data to detect "activity of concern"; new technology that aims to automatically detect terrorist content before it hits the web; and proposals to lift a ban on new police recruits carrying Tasers.
Other options being explored include possible action to prevent the "malicious" use of hire vehicles and greater use of orders issued to restrict suspects who cannot be prosecuted or deported.