The Manchester bomber has been pictured moments before Monday's massacre, as a wave of arrests fuelled hope that further terror threats had been quelled.
Police issued CCTV stills of Salman Abedi, bespectacled and casually clothed, in a plea for information about his movements between May 18 and the attack.
A matter of hours after he was captured on camera, the 22-year-old was dead, having inflicted an outrage on a pop concert attended predominantly by young girls.
The huge police operation that followed saw raids in several cities as counter-terror efforts were focused on cornering his suspected criminal ring.
The de-escalation in the country's threat level from "critical" suggests confidence that the police operations conducted in the last week have headed off any immediate threats.
In the latest raids a 20-year-old and a 22-year-old were detained on suspicion of terror offences in Cheetham Hill, near Manchester city centre.
It came amid searches at a separate property in Cheetham Hill and an address in the Longsight area in south Manchester, while a road in Moss Side was evacuated by officers searching a home there.
Monday night's suicide bombing left 22 dead, including seven children, and scores of others injured - the worst atrocity to hit Britain since the July 7 attacks in 2005.
Speaking after a meeting of the Government's Cobra emergency committee, Mrs May said the decision to lower the threat level followed "a significant amount of police activity" over the last 24 hours.
She said: "The public should be clear about what this means - a threat level of severe means an attack is highly likely. The country should remain vigilant."
The terror threat had been at critical for the first time in a decade.
Greater Manchester Police Chief Constable Ian Hopkins hailed "significant progress" in the investigation, following disclosures that "key players" of Abedi's suspected terror network had been captured.
Mrs May also said Operation Temperer, allowing the military to be deployed to protect key sites, will be reined in after the bank holiday.
She said: "To provide maximum reassurance to the public, Operation Temperer will continue to operate until the bank holiday concludes.
"Then, from midnight on Monday onwards, there will be a well-planned and gradual withdrawal of members of the armed forces, who will return to normal duties."
An unprecedented security operation was in full swing on Saturday as tens of thousands of people attended major spectacles including the FA Cup Final at Wembley and the Premiership Rugby Final at Twickenham.
Arrangements have been reviewed at more than 1,300 events, while the pool of armed officers available to be deployed around the country has been boosted by 1,100 after military personnel were drafted in to cover guard posts at high-profile locations.
With Manchester still in the throes of mourning, a vigil was announced for victim Martyn Hett by his family, to take place from 7pm at Heaton Moor Park in Stockport on Sunday.
The 29-year-old PR manager was remembered fondly in the wake of the bombing for his tireless enthusiasm, captured in the viral hashtag #BeMoreMartyn.