Police want to speak to the Manchester Arena bomber's brother and believe "other people" may be involved in the attack.
Salman Abedi blew himself up as crowds were leaving an Ariana Grande concert, killing 22 people including seven children.
His brother, Hashem Abedi, was arrested in Tripoli after the 22 May atrocity and Greater Manchester Police are now liaising with the Crown Prosecution Service and Libyan authorities to secure access to him.
Chief Superintendent Russ Jackson, head of counter terrorism in the North West, said he did not believe it was a "large network" that was involved in the bombing.
"We don't have evidence that there were other suicide bombers, we do suspect others were aware or complicit in their knowledge of the attack," he said.
"We do believe there may be other arrests in the future."
Three million files and 16,000 hours of CCTV have been collected as police have sought to detail Abedi's movements in the days and weeks before the attack, and establish any terrorist connections he may have.
Police believe they have a clear picture of Abedi's movements but are still looking at how he acquired the skills to build his bomb.
They said he appeared to have spent "many months" planning the atrocity - but had left no note or video explaining his motivations for the attack.
A landfill site is still being searched for a blue suitcase connected to Abedi.
Meanwhile, Grande has paid tribute to the youngest victim of the bombing after performing in Buenos Aires.
"Saffie we're (thinking) of you baby," she wrote alongside a birthday cake emoji on her Instagram account.
Saffie Roussos, who was leaving the concert with her family when the bomb went off, would have turned nine on 4 July.