The May 22 attack at an Ariana Grande concert killed 22 people as well as bomber Salman Abedi, a Briton of Libyan descent. Police are trying to uncover clues about a suspected network that supported him.
Greater Manchester Police released photos of a white Nissan Micra seized in Manchester, saying "significant evidence" had been found inside.
Police appealed for anyone who saw the car between April 13, when its previous owner sold it, and April 15 - when Abedi left the UK to contact detectives.
Police also released a photo of a sports bag with a San Francisco 49ers logo, saying anyone who recognized it should contact them.
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Police arrested a total of 18 people over the attack, but have released nine of them without charge. Among those released was Ismail Abedi, the dead bomber's brother, arrested a day after the attack. A 20-year-old man who was arrested May 27 was freed without charge today. His name was not released.
The Islamic State group has claimed responsibility for the attack.