Manchester Arena suicide bomber Salman Abedi may have stored items used to assemble the device in a Nissan Micra, police have said.
Greater Manchester Police said "significant evidence" was found in the white car, which has an "R" registration plate and was seized from Devell House in Rusholme on Friday.
The force issued photos of the car and a red and black holdall and appealed for anyone who recognised either to come forward.
Detective Chief Superintendent Russ Jackson said police were continuing to track the movements Abedi made in the days leading up to the May 22 attack, which killed 22 people, including seven children.
He said: "Our investigation has also revealed that Abedi made repeated trips to and from this car between 18 and 22 May 2017 and we believe he was taking items from the car to help assemble the device.
"The car was sold by a previous keeper on 13 April 2017.
"Abedi left the country on 15 April and it is vital that we understand what happened to this car during these few days between 13 and 15 April.
"We need to know who was in the car and where the car went. You may have seen the car and not realised at the time but it could have stopped at a newsagents or a supermarket - did you see the car and its occupants during these dates?"
Mr Jackson said it was "vital" officers exhausted all lines of enquiry to establish how other people were involved in planning the attack.
He added: "We also appeal to anyone who knew Abedi or his close associates to come forward. Did you notice anything suspicious about him or people he associated with? This information could be vital in piecing together what happened."
The police appeal came as a ninth person was released after being arrested in connection with the attack.
The 20-year-old man had been arrested in Cheetham Hill on May 27.
Suicide bomber Abedi's brother Ismail, 23, was released on Monday evening.
He was arrested in the street in Chorlton on Tuesday May 23, the day after the terror attack on Manchester Arena, which killed 22 people, including seven children.
On Tuesday, a police officer outside Ismail Abedi's flat in nearby Whalley Range told reporters that the property was still "an active crime scene".
Nine men remain in custody for questioning in connection with the attack.
Two of Salman Abedi's cousins, Isaac Forjani, 24, and Abz Forjani, 21, both from Fallowfield, were among those released last week.
A third cousin, aged 18, is still being questioned.
Salman Abedi, 22, whose family are of Libyan origin, died after detonating an improvised explosive device as people began leaving a show by US singer Ariana Grande.
His father, Ramadan Abedi, was arrested in Tripoli on May 24, along with Salman's younger brother Hashim, 20, who Libyan security forces said were "aware of all the details" of the attack.
Islamic State claimed responsibility for the atrocity.