Nikolas Cruz has been charged with 17 counts of murder after the mass shooting at a Florida high school in a case which could see him executed.
The 19-year-old will plead guilty to the attack at Marjory Stoneman Douglas High School if he can avoid the death penalty, his public defender has said.
Broward County officials have not announced if they will seek capital punishment.
Cruz admitted to police that he took an AR-15 rifle into the high school on 14 February and started shooting.
Cruz is being held in solitary confinement and has been described as co-operative, although police say he avoids eye contact and "often sits with a blank stare".
At the grand jury hearing, the couple James and Kimberly Snead who had taken in Cruz after his mother died, said they were shocked by his actions and did not predict his capacity to kill, Sky News reported.