CCTV images showing suicide bomber Salman Abedi on the night he attacked the Manchester Arena killing 22 people have been released by police.
Fourteen locations are being searched and 13 men are in custody on suspicion of terror offences.
Abedi's identity was known within two hours of Monday's attack, police said.
The latest person to be arrested was a 19-year-old man in the Gorton area of Manchester. A 25-year-old man was held earlier in the Old Trafford area.
Searches are ongoing in the city's Rusholme and Gorton areas.
Armed police continue to guard hundreds of events across the UK over the bank holiday weekend.
Additional security measures were in place for the Great Manchester Run, where tens of thousands of runners took part, including Greater Manchester Mayor Andy Burnham.
The second day of Radio 1's Big Weekend in Hull is also subject to extra security.
The UK threat level has been reduced from critical to severe and soldiers deployed to support the police will be gradually withdrawn from Monday night.
Prime Minister Theresa May made the announcement after leading a meeting of the government's emergency committee Cobra on Saturday morning.
Mrs May said significant activity by the police during the last 24 hours had led to the threat being reduced.
It is unclear where the newly released images of Abedi were taken and from what time on Monday evening.
Greater Manchester Police have made an appeal to the public for information about the bomber's movements since 18 May, when he returned to the UK.
Abedi's city centre flat was one of the last places he went - and where he may have made the final touches to his explosive device - before going to the arena, police have said.
Police said the investigation - which is "making good progress" - has around 1,000 people working on it and hundreds more officers involved in the security across Greater Manchester.
In total 15 people have been arrested in Greater Manchester, Warwickshire and Merseyside - but a woman and a 16-year-old boy were later released without charge.
Seven children were among those who died when the Manchester-born 22-year-old detonated the bomb on Monday night, at the end of a concert by US singer Ariana Grande.
A vigil is being held for 29-year-old victim Martyn Hett at Heaton Moor Park in Stockport on Sunday evening.
And the family of student Georgina Callander, from Lancashire, have said she was "as beautiful on the inside as she was on the outside, with a smile that was never ending. She always had big hugs for everyone and her life was blooming like the flowers she loved".
In a statement issued through Greater Manchester Police, they said her life was "taken away after 18 short years by evil, evil men prepared to ruin lives and destroy families, for what?"
They added: "I wish I could say that Georgina is one of the last to die in this way but unless our government opens its eyes we know we are only another in a long line of parents on a list that continues to grow."
Anyone with information is asked to call the anti-terrorist hotline on 0800 789321. You can upload images or footage that might be of assistance to ukpoliceimageappeal.co.uk.