Investigators in Georgia on Thursday sought to piece together how a teenager obtained the assault rifle he allegedly used to carry out a mass shooting at his school and whether there were any additional warning signs after authorities visited his home a year ago.

The student, identified as Colt Gray, 14, opened fire on Wednesday with an AR platform-style weapon at Apalachee High School in Winder, Georgia, killing two students and two teachers and wounding nine, law enforcement officials said.

Gray was interviewed by law enforcement last year after he made online threats about carrying out a school shooting, according to investigators. His father, who also was interviewed, told officials he had hunting guns in the house but his son did not have access to them.

The shooter’s ability to obtain the semiautomatic rifle, any signs warning he would actually carry out a shooting and his motive are focuses for investigators digging into the United States’ first mass campus shooting since the start of the school year.

Gray was taken into custody shortly after the shooting. He will be charged and tried as an adult, said Chris Hosey, director of the Georgia Bureau of Investigation.

The bureau on Thursday did not yet have information on when Gray would make his first court appearance.

Officials identified those killed as two 14-year-old students, Mason Schermerhorn and Christian Angulo, and two teachers, Richard Aspinwall, 39, and Christina Irimie, 53. One teacher and eight students wounded in the attack remained hospitalised, MSNBC reported early on Thursday.

The shooting revived both the national debate about gun control and the outpouring of grief that follows in a country where such attacks occur with some regularity.

People in Winder, a city of 18,000 some 80 kilometres northeast of Atlanta, gathered in a park for a prayer vigil on Wednesday night.

The shooting was the first planned attack at a school this fall, said David Riedman, who runs the K-12 School Shooting Database. Apalachee students returned to school last month; many other students in the US are returning this week.

The US has seen hundreds of shootings inside schools and colleges in the past two decades, with the deadliest resulting in over 30 deaths at Virginia Tech in 2007.

The carnage has intensified the pitched debate over gun laws and the US Constitution’s Second Amendment, which enshrines the right “to keep and bear arms.”