Several US media outlets have reported that a Batman mask and poster were in the flat, but police have not confirmed this.
Booby traps connected to explosives had prevented police from entering it for two days until they made the devices safe.
Police said the suspect had acted with "calculation and deliberation", taking ammunition deliveries for months.
James Holmes is due to appear in court on Monday.
Authorities have established no terrorism link, nor any motive. Mr Holmes had no criminal record other than a speeding fine.
It has emerged that Mr Holmes tried to join a Colorado gun range last month but owner Glenn Rotkovic told Associated Press he warned staff not to accept him after hearing a "bizarre - guttural, freakish at best" message left by Mr Holmes on his voicemail.
The Arapahoe county coroner Michael Dobersen has released the names of the dead.
He said all the deaths were related to gunshot wounds.
The dead include a six-year-old girl and two US military servicemen.
Mayor Hogan told a memorial service for a young victim on Saturday that the community in Aurora was still reeling from the "unbelievable tragedy".
"But I think, as is evidenced by everyone who is here today, all those who were here yesterday, and all the messages of support that we have received from literally around the world, we can I think now start this process of grieving and healing."