Residents are being evacuated in Manchester after police cordoned off a suspect car possibly linked to Salman Abedi.
Greater Manchester Police said the white Nissan Micra "may be significant to the investigation" as the operation continued.
Authorities were urging people to keep away from a large security cordon around the scene in Rusholme Place.
Detective Chief Superintendent Russ Jackson, head of the North West Counter Terrorism Unit, said: "This is potentially a significant development in the investigation.
"We are very interested in anything people can tell us about the movements of this car, and who was in it, over the past months.
"We are also interested in any information about who may have had access tot he car or who may have gone to and from it."
The car is outside a block of flats called Devell House in Rusholme, the area where Abedi returned repeatedly carrying a blue suitcase in the run-up to the bombing.
Manchester Royal Infirmary has been unaffected by the operation, but its Ronald McDonald House centre for the families of child patients has been evacuated, as well as surrounding homes and student accommodation.
The cordon was continuing to widen more than an hour after police announced the search, with a nearby pub and shops being shut along Wilmslow Road.
Authorities said the cordon was a "precaution" and that people would temporarily be relocated.
Police are attempting to piece together the bomber's movements from when he flew back from Libya on 18 May and the attack four days later.
New CCTV images have been released showing Abedi moving around Manchester during the period, sometimes carrying a blue suitcase investigators also want to trace.
On Thursday, Det Ch Supt Jackson said he purchased parts for the bomb after arriving back in the UK, adding: “What we still need to understand is if he had any of the bomb parts in his possession before he went out of the country.
“This is why we are tracking his movements so carefully.”
The detective confirmed that searches in the Banff Road area earlier this week were sparked by information given by members of the public, but despite house-to-house enquiries, police are “still not satisfied why Abedi went there”.
“It is vital we understand exactly where he went there and who he spoke to in these final days before the attack,” he added.
“We need your help as we piece together what happened in the lead up to the attack and if Abedi was helped by anyone.”
Police previously confirmed that the attacker purchased many of the “core components” himself and made many movements alone in the four days before the attack.
But investigators are still probing the possibility of a wider network supporting the attack, which killed 22 people and injured dozens more at Manchester Arena.
Greater Manchester Police confirmed that 10 men, aged between 18 and 44, remain in custody on suspicion of terror offences.
Two of Abedi’s cousins, his former landlord and a 15-year-old boy are among six people who have been released without charge so far in the investigation.
Anyone with information is asked to call the anti-terror hotline anonymously on 0800 789321 or send images and footages to police by visiting the UK Police Image Appeal website.
