Hall County Sheriff Gerald Couch told reporters yesterday that a search of Harbour's belongings revealed handwritten documents praising the Parkland killer, calling him a "hero" who gave him "courage and confidence."
Those revelations were disturbing and "hate-filled," Couch said.
Investigators recovered an arms cache from the 2003 Buick Century driven by Harbour, including three 9mm semiautomatic pistols, a .22-calibre bolt-action rifle, a 12-gauge shotgun and more than 3,400 rounds of ammunition that was mostly for the rifle, Couch said.
The victims were all struck by pistol fire, and authorities said they think it was the only weapon Harbour fired. A total of seven cars were struck by gunfire, authorities said.
Couch recounted the brief but terrifying few moments along Highway 365. A woman called emergency services to report that her husband was shot while driving 12 minutes before noon, and other calls of gunfire hitting vehicles soon flooded in.
A deputy questioning one victim saw Harbour's beige Buick pull out, and ran towards it with a gun drawn. Harbour saw the deputy and sped away.
The pursuit by deputies and Georgia State Patrol was brief, with speeds up to 130km/h. Harbour put a pistol to his head and pulled the trigger while the car was still moving. It continued on a driverless path for "quite some distance" before coming to a stop, Couch said.
A dash camera recorded the shooting, he said.
Harbour's firing position alongside the road was probably chosen because it was isolated and concealed, authorities said. But a coincidence led them to detail his movements before authorities arrived. A trail camera strapped to a tree, probably left by a hunter to document paths of movement of game such as deer, was nearby and took photos of Harbour readying his position.
He wore two pistol holsters at that moment, Couch said. Authorities recovered 17 shell casings at the scene.
Couch said the gunman had no history of criminal or notable behaviour. His mother told authorities that Harbour was "mild-mannered and quiet."
Harbour's motive was unclear, authorities said, but Couch speculated that it was notoriety.
"He had the weapons, he had the ammunition, and obviously he had the will to inflict a lot of pain and a lot of hate," Couch said.