Witnesses say his rampage was only stopped after two people tackled him, and a local sheriff is said he may not have acted alone.
According to the FBI and the justice department, Loughner left a trail of videos and Internet postings that hinted at the havoc he would release.
A look at Loughner's MySpace page suggests he was preparing for a violent onslaught. A few hours before the shootings, he allegedly posted a message entitled "Goodbye friends". It began, "Dear friends ... please don't be mad at me."
Violence and disgust with government appear to have animated Loughner. A YouTube video posting, one of his favorites, "Let the bodies hit the floor," shows a U.S. flag burning.
In another YouTube posting, Loughner writes, "Current government officials are in power for their currency."
Loughner appeared paranoid, suggesting that government was monitoring his communications.
"I know who's listening," he wrote.
He also discussed mind control and brain-washing methods.
Federal law enforcement officials plan to trace the firearm used in the shooting. Sources described it as a semi-automatic pistol with an extended clip.
The FBI has begun to dissect the shooter's background and to look for any possible accomplices.
"A comprehensive investigation is currently under way," U.S. President Barack Obama said.
The U.S. Capitol Police are also assisting in the investigation, and plans were under way to step up security for congressional leaders.
The shooting elicited a rare statement from the head of the U.S. Supreme Court, John Roberts.
"Chief Judge Roll's death is a somber reminder of the importance of the rule of law and the sacrifices of those who work to secure it," the top judge in the land said.