More than 100 state troopers and FBI officers are searching for a man who killed three people in an Atlanta courthouse Friday.

The suspect, 33-year-old Brian Nichols, remains at large causing fear that he might strike again.

Police say he may be armed and extremely dangerous and should not be approached. A $60,000US reward is offered for information leading to his arrest.

An undated file photo of Brian Nichols, 33, who is suspected by police to have shot and killed Superior Court Judge Rowland Barnes at the Fulton County courthouse in Atlanta on Friday, March 11, 2005. (AP Photo)

Witnesses say Nichols grabbed the gun of a sheriff's sergeant and opened fire inside the courthouse, killing Judge Rowland Barnes and his court reporter. A deputy who had also been shot later died in hospital.

A fourth person, a woman, was in critical condition with a head wound she received in the incident, which happened at about 9 a.m. local time.

Among the cases handled by Barnes was the 2003 car crash involving Dany Heatley that killed the NHL hockey star's 25-year-old teammate Dan Snyder. Heatley was sentenced to three years' probation in early February.

Witnesses said the male suspect, who was in court facing a retrial on charges of rape, sodomy, burglary, and false imprisonment, took the gun from the sheriff's deputy and began shooting inside the Fulton County Courthouse.

Police said the judge was shot on the eighth floor of the courthouse, while one deputy was shot on a street corner just outside the building.

The judges in the building were locked in their chambers. The courthouse and other buildings in downtown Atlanta were on lockdown.

On Thursday, investigators found sharp objects in Nichols' shoes. His last known job was working as a computer technician for UPS.

Police say Nichols escaped by pistol-whipping a reporter and driving away with his Honda.