Pictures of the Manchester bomber on his way to the arena where he carried out Britain’s worst terrorist attack in 12 years have been released by police. CCTV footage shows Salman Abedi, 22, shortly before his suicide attack on an Ariana Grande concert at Manchester arena on Monday night.

The CCTV images show Abedi in glasses, wearing a black puffa jacket, baseball cap, jeans and trainers and carrying the rucksack containing the device that killed 22 people and injured 119 others.

Police believe he assembled his deadly bomb in a short-term let in a mansion block on Granby Row in the city centre, near Canal Street. The flat is one of 17 locations searched by forensic experts in the past few days. Fourteen searches were continuing on Saturday night.

Salman Abedi. Photograph: Greater Manchester Police/PA

Detectives know that Abedi returned to the UK from Libya on 18 May, four days before the bombing. They hope the images will jog the memories of people who may have seen him in the run-up to the attack.

In a statement released on Saturday night, hours after the terrorism threat level in the UK was reduced from critical to severe, Greater Manchester’s police chief, Ian Hopkins, and its deputy assistant commissioner, Neil Basu, said the investigation was making good progress.

Thirteen people have so far been arrested on suspicion of terrorism offences, police said. A thousand people are involved in the investigation to Abedi’s network.

“In the past five days we have gathered significant information about Abedi, his associates, his finances, the places he had been, how the device was built and the wider conspiracy. As a result of the arrests and searches which have taken place, we now have many further lines of inquiry. We already have more than 1,500 actions we are pursuing,” the officers said.

“Since Monday night, Greater Manchester police and Counter Terrorism Policing North-West have been working closely with the national counter-terrorism policing network on what continues to be a large-scale and fast-moving investigation.

“The investigation is making good progress and we know one of the last places Abedi went was the city centre flat, and from there he left to make his way to the Manchester Arena. The flat is highly relevant as a location which we believe may be the final assembly place for the device.”

The latest arrests took place in the early hours of Saturday morning during a raid in Cheetham Hill, a multicultural area of north Manchester. Two men aged 20 and 22 were taken into custody. Neighbours identified Yahya and Mohamed Werfalli, aged 20 and 22, as two of the occupants of the raided house. They were said to be of Libyan descent and part of the same friendship groups as the Manchester bomber.

Shortly after those arrests, armed officers arrived at a terraced house in Boscombe Street, on the Moss Side/Rusholme border, near the site of Manchester City’s old Maine Road ground. Some residents were evacuated as bomb disposal experts from the army carried out investigations in the property.

In their joint statement, the senior officers revealed details on how the investigation unfolded following the bomb, which detonated around 10.30pm on Monday just as the concert finished.

Within an hour, GMP had set up a specialist counter-terrorism control room, with a first priority of identifying the attacker. “Specialist counter-terrorist forensic teams were sent to try and identify the attacker and within two hours his identity was known. With this information, officers could begin to establish his movements to try and understand if anyone else was linked or any more attacks planned,” the statement said.

Firearms officers from across the country were deployed to support GMP in case of a further attack. More officers from around the national counter-terrorism policing network across the UK soon arrived. “By early Tuesday morning there was an established pattern, with all officers and staff working on the response to the attack in day and night shifts of approximately 14 hours each,” the senior officers said.

They added: “The whole team are working round the clock. We have around 1,000 people involved in the investigation alone. In addition, there are hundreds of officers and staff from Greater Manchester Police and other forces involved in the security around Greater Manchester.

“This is still a live investigation, which is not slowing down. Our priorities are to understand the run-up to this terrible event and to understand if more people were involved in planning this attack.”

Anyone with information should call the Anti-Terrorist Hotline in confidence on 0800 789321. If you have any images or footage that you believe can assist, then upload them to Ukpoliceimageappeal.co.uk or Ukpoliceimageappeal.com

• This article was amended on 29 May 2017. An earlier version said the Granby Row flat was an Airbnb let. This has since been proved to be incorrect.