Chief Rick Kastigar with the Pima County Sheriff's Office told The Associated Press that an 18-year-old found the bag Thursday morning in a neighborhood near where the suspect lives. He went to get another man, who looked inside and saw ammunition.
Authorities have said the suspect, Jared Loughner, and his father got into a confrontation over the bag at their home in the hours before the shooting.
This detail is the latest in a series of snapshots emerging about the final hours before Loughner allegedly opened fire with a semi-automatic Glock handgun in a crowded Tucson Safeway parking lot.
Authorities said Wednesday that a wildlife officer pulled over the suspect in the assassination attempt against an Arizona congresswoman less than three hours before the deadly attack.
Lougher, 22, ran a red light but was let off with a warning at 7:30 a.m. Saturday, the Arizona Game and Fish Department said. The officer took Loughner's driver's license and vehicle registration information but found no outstanding warrants on Loughner or his vehicle.
Wildlife officers have full authority to make traffic stops, although they usually don't do so unless public safety is at risk, the department said in a news release.
Jim Paxon, chief information officer for Arizona Game and Fish told CBS News that the stop took place at Interstate 10 and an unnamed cross street, "several miles" from the Safeway store where the shootings took place.
The night before the shooting, Friday, Jan. 7, Loughner slept at an area motel. He apparently drove to the motel in the Chevy Nova now parked at the Loughner home. Police have since searched the vehicle thoroughly.
At 2:33 a.m. Saturday, Loughner visited a convenience store less than a mile from his house, CBS News reports. Alone, he bought a doughnut, a soft drink and several energy bars. He did not buy alcohol.
Surveillance video of Loughner at the Catalina Mart has been given to the FBI, which wanted the video to see if he had any accomplices, CBS News reports. The FBI scanned Loughner's credit card purchases to track his whereabouts before the shooting.
The morning of the shooting, Loughner went to two WalMart stores to buy ammunition. He left the first store because the clerk was too slow. He bought the ammo at the second store.
Either right before or right after buying the ammo, Loughner was pulled over for running a red light and released, in the earlier described incident.
After being pulled over, Loughner went home and had a confrontation with his father, Randy Loughner.
Loughner, who was chasing his son after seeing him remove a black bag from the trunk of a family car, said Rick Kastigar, chief of the Pima County Sheriffs Department's investigations bureau. Randy Loughner had apparently asked after the bag, and his son then fled.
After the run-in with his dad, he took a cab from a convenience store to the Safeway.
The suspect was "moving around quite a bit, going from location to location," the morning of the shooting, the FBI said recently, as they continue to develop a time line of the suspects movements prior to the shooting and the weeks and months before as well.
The sheriff's deputies who swarmed the Loughners' house removed what they describe as evidence Loughner was targeting Giffords. Among the handwritten notes was one with the words "Die, bitch," which authorities told The Associated Press they believe was a reference to Giffords.
Investigators with the Pima County Sheriff's Department previously said they found handwritten notes in Loughner's safe reading "I planned ahead," "My assassination" and "Giffords." Capt. Chris Nanos said all the writings were either in an envelope or on a form letter Giffords' office sent him in 2007 after he signed in at one of her "Congress on Your Corner" events - the same kind of gathering where the massacre occurred.
In addition to the new details about the hours before the shooting, interviews with those who knew Loughner or his family painted a picture of a young loner who tried to fit in, but was.
Before everything fell apart, he went through the motions as many young men do nowadays: Living at home with his parents, working low-wage jobs at big brand stores and volunteering time doing things he liked.
None of it worked. His relationship with his parents was strained. He clashed with co-workers and police. And he couldn't follow the rules at an animal shelter where he spent some time.