A gunman walked into the state Capitol Monday and fatally shot an unarmed security guard, then walked out, put his gun in a car trunk and sped off as onlookers yelled for help.

Police say the shooter matched the description of a man involved in an apparent robbery attempt about an hour earlier at a nearby military surplus store and also in the theft of a shotgun from the same store a week before.

Authorities identified the suspect as Derek W. Potts (search), 24, and police said the Sangamon County state's attorney's office had obtained an arrest warrant accusing Potts of murder, burglary and more.

Col. Larry Schmidt (search), chief deputy director of the Secretary of State's police force, said he could not discuss what led police to conclude Potts is the killer. He said there was no indication Potts has ever been in trouble with the law.

Acting on a tip, they searched Pott's apartment late Monday night, but he was not home. Police surrounded the apartment building near the Capitol, evacuating the tenants and blocking off traffic for hours, before obtaining the search warrant. A car similar to the one driven by the killer was in the parking lot.