Jared Lee Loughner, charged with killing six people and trying to assassinate a US congresswoman in Arizona, checked into a motel, bought bullets and had photographs developed on the night before the shooting, police said on Friday.
The New York Times said the roll of film had "multiple photos" of the 22-year-old college dropout posing in red G-string underwear with a Glock semi-automatic pistol.
FBI sources told Reuters they could neither confirm nor deny the report.
Loughner is charged with five federal counts for allegedly firing into a crowd outside a grocery store in Tucson on January 8 at a public event held by Representative Gabrielle Giffords.
The rampage has reignited debates in the United States about the implications of heated political rhetoric and the relative ease of legally acquiring weapons.
The six dead people included a 9-year-old girl and a federal judge, John Roll, whose private funeral on Friday was attended by hundreds of mourners under tight security.
Giffords, one of 13 people wounded, remains in critical condition after a bullet tore through her brain but doctors said she is regaining some physical function and awareness.
"We're actually confident that she's making some progress now," Dr Michael Lemole told reporters at the University Medical Centre in Tucson.
Giffords, 40, is getting better at following simple voice commands and is opening her eyes more frequently, he said.
Doctors were "very encouraged that she's continuing to make all the right moves in the right direction," Lemole said.
The Pima County Sheriff's Department, which arrested Loughner, released a timeline of his activities on the Friday evening and early hours of Saturday before the shooting.
It detailed how he checked into a motel, bought ammunition and a backpack and dropped off film at a drugstore, collecting the developed photos several hours later, shortly before 2:30am.
The sheriff's department said the FBI had the photos and security video from businesses near the scene of the shooting.
Loughner also posted a message on the Myspace social networking website saying "Farewell friends."
The sheriff's department said he was warned verbally by an officer for running a stoplight at 7:30am before returning home, where he took a black bag from his car and argued with his father.
At 9:41am, he took a taxi to the grocery store where, just after 10am, he allegedly shot into the crowd at the meeting with Giffords before being wrestled to the ground.
The sheriff's department said Loughner was carrying two 15-round magazines of ammunition, a knife, some money, a credit card and his Arizona driver's license.
