Prosecutors said the suspect, a 24-year-old college dropout, has a history of mental illness including manic depression, and has been off his medication. They are reviewing whether to seek the death penalty.
"This was an extremely violent crime," prosecutor John Schmidt told a judge in successfully seeking a $20 million bond for Derek W. Potts.
Potts was charged with first-degree murder, burglary involving a gun from a military surplus store, and gun violations.
Defense attorney Brian Otwell said mental health would likely be an issue in the case. "He's not familiar with the criminal justice system," he said. "He's scared and frightened."
Potts was unarmed when officers spotted him just after dawn, Police Chief Don Kliment said.
"The guy more or less surrendered," said Phillip Giger, 50, who said he was making coffee when he heard car doors slam and looked out to see a man talking to police. "They were so quiet my dog didn't even notice it."
Police had said they knew of no motive for the shooting or any connection between Potts and the victim, William Wozniak, a father of two who was in his 18th year as a Capitol guard.
Wozniak, 51, was working at the Capitol's main entrance Monday afternoon when a gunman marched inside and shot him once in the chest, then walked back out, stowed his gun in a car trunk and drove away as witnesses shouted for help.
The Capitol has no metal detectors, and its security guards are unarmed.
Gov. Rod Blagojevich pledged Tuesday to quickly install metal detectors and also look at other safety measures, such as arming the guards.
"There's no need to wait," he said at a Statehouse news conference where the charges against Potts were announced. "There's an urgency to get this done."
Potts, originally from the small town of Olney, has some traffic offenses and two misdemeanor drug charges on his record, one involving marijuana and the other alcohol. Both resulted in fines and court supervision, officials said.
"He didn't have any unusual circumstances, as far as the criminal cases we had," said Richland County State's Attorney Kaye DeSelms. "There were no red flags."
On Sept. 14, police believe Potts was involved in a robbery at a military surplus store in Springfield where a shotgun was stolen.
On Monday, someone matching Potts' description walked in the same military surplus store carrying a 12-gauge shotgun and demanded a high-powered rifle, but fled when the owner recognized him, police said.
A little over an hour later, Wozniak was shot.
By John O'Connor