The Manchester Arena bomber carried the device through city centre streets for "several hours" before the attack, police believe.
Salman Abedi detonated the home-made bomb, with metal nuts used as shrapnel, at an Ariana Grande concert on 22 May.
Abedi was not part of a larger network, the head of the North West Counter Terrorism unit has said.
Ch Supt Russ Jackson said others may have been "aware or complicit" in the attack that killed 22 people.
He said further arrests may follow.
More than 250 people were hurt in the blast and have injuries ranging from paralysis and loss of limbs to internal and facial injuries, he said.
Abedi was walking around Manchester city centre with the bomb before he detonated it, but police do not believe he had any target other than the Arena in mind.
The bomb had a "devastating" impact and gouged out a section of the concrete floor.
Ch Supt Jackson confirmed officers want to interview the bomber's younger brother, Hashem Abedi, who continues to be held by special deterrence forces in Tripoli.
He would not comment on whether officers had travelled to the country, but said the force was engaged with the Libyan authorities and the Crown Prosecution Service.
Briefing reporters at the headquarters of Greater Manchester Police on Thursday, Ch Supt Jackson said it was "hard to get inside [Abedi's] head" in terms of how he was radicalised.
"Salman Abedi travelled to Libya a number of times in his life. What we are looking at is the number of ways he learned the skills to build the device," he said.
Abedi left no note or video explaining his actions, he said.
Ch Supt Jackson said officers were still searching for a blue suitcase in a landfill site, and this was a "key line in the inquiry".
The investigation was expected to continue for "many, many months to come", he said.
It will not be quick, as police have 16,000 hours of CCTV footage and 755 statements to analyse, he added.
"Significant forensic evidence" was also found in a Nissan Micra in Rusholme, Greater Manchester.
Police earlier said Abedi may have used items stored in his car "to help assemble the device" he used to kill.
Ch Supt Jackson said "digital exhibits" containing more than three million files and 15 terabytes of data have also been recovered.
Asked if Abedi was involved in gang activity in Manchester, Ch Supt Jackson said he may have known "people who would be identified as being in gangs", but there was no suggestion of a link to gang activity in the attack.
Police are not looking for any particular suspects.
Ch Supt Jackson said people who were arrested then released without charge had "not reached the threshold" [for further investigation].
"It should be noted that terrorism offences do not carry the option of bail. They can only be released without charge."
Forensic officers who were working at the arena in the days after the attack laid roses next to name plates at each spot where the 22 victims were killed, Ch Supt Jackson revealed.