The stop occured several miles from the scene of the shootings, which left Rep. Gabrielle Giffords (D-Ariz.) critically wounded. Law enforcement officials, speaking on the condition of anonymity, said Loughner was running errands, which included trying to buy ammunition at a Wal-Mart.
Later that morning, according to law enforcement officials, Loughner's father, Randy, confronted him outside their home as Jared Loughner removed a black bag from the trunk of a family car. Jared Loughner grabbed the bag and ran, and his father chased after him in his car but lost sight of him, the sources said.
They said that the bag remains unaccounted for. Investigators are attempting to determine its significance.
Investigators think Loughner, 22, did not have enough income to buy the Glock 19 semiautomatic handgun, four magazines, ammunition and knife he allegedly carried to Giffords's event at a Tucson supermarket, the officials said. They estimated the cost at close to $1,000.
The FBI and Pima County Sheriff's Department investigators are examining the Loughner family's financial records, as well as Jared Loughner's telephone, Internet and e-mail records, as they try to determine where the money to buy the weapons came from, said the officials, who spoke on the condition of anonymity because they are not authorized to talk about an ongoing investigation.
When authorities searched the house of Loughner's parents on the day of the shootings, they found a shotgun that Jared Loughner had bought the year before at the same gun store where he purchased the Glock. They also found disturbing writings, including a note that said "[expletive] you pigs."
Although the shooting was witnessed by scores of people, the video footage could be key evidence, investigators said. It shows the gunman rushing forward, with his Glock at his side at first, and moving toward a table where Giffords is standing, one law enforcement official said. "While the video of the shooting was not perfect, it was pretty close," the official said.
Also Wednesday, Pima Community College released records showing that teachers called campus police five times from January to September 2010. Though Loughner saw counselors on at least two occasions and records show campus police registered their concern that he may be mentally ill, the records indicate that he received no referral for mental health evaluation or treatment.
"He seems to have difficulty understanding how his actions impact others, yet very attuned to his unique ideology that is not always homogenous," counselor DeLisa Siddall wrote in an e-mail released Wednesday. "Since his resolution was to remain silent in class and successfully complete the course, I had no grounds to keep him out of class."
But after an outburst in September over his biology grade, campus police were clearly becoming worried. They wrote that he was unable to communicate his thoughts clearly when they spoke to him. "I noticed during my questioning that Loughner's head was constantly tilted to the left and his eyes were jittery and looking up and to the left," one officer wrote.
On Sept. 29, officers found a YouTube video by Loughner that they said violated the school's code of conduct. Later that evening, two officers, with two backup officers called to be in the area, went to Loughner's home to deliver his suspension letter. Randy Loughner let the officers in through the garage after putting away the family's dogs, the report says. His son "held a constant trance of staring" while officers read the suspension letter. After an hour, Jared Loughner spoke: "I realize now that this is all a scam," he said.