The deadly attack by a student at Debsirin Nonthaburi School appears to have resulted from a combination of factors such as family problems, issues at school, academic pressure, conflicts with friends and exposure to violent online content, police said on Sunday.
Police had questioned 17 witnesses - teachers, classmates and close friends of the student. Evidence suggested Friday’s shooting was not triggered by a single issue, police said.
Police had learned that the 14-year-old's parents had separated when he was very young. He had been raised by his grandparents since the age of two.
The grandparents were earlier reported to be his first victims on Friday, killed with the grandfather's handgun.
Police were also looking into issues related to school life, friendships and reports of possible bullying or conflict with teachers.
Prime Minister Anutin Charnvirakul on Saturday asked the public not to speculate that bullying alone was responsible for the tragedy.
Police said the suspect's old mobile phone and computer showed that he had viewed violent online content and developed an interest in guns in the past year or two.
The young teenager bought a BB gun online last year and later took it to school, where it was confiscated by teachers.
He had also previously taken a knife to school in his school bag.
During the two-hour case review meeting, police divided the attack into four stages as follows.
Stage 1: Initial attack
The suspect arrived at school and later fatally shot a teacher in Classroom 3/8 on the sixth floor at 9.53am.
Stage 2: Attacks on the sixth and fifth floors
After shooting the first teacher he moved to Room 565, where two teachers were killed and one student also shot.
At 9.54am, he descended to the fifth floor, inserted a fresh magazine in his gun and attempted to continue shooting - but his gun reportedly jammed.
He then moved to the fourth floor and shot the deputy director and the director's secretary. Police arrived at 10.02am.
Stage 3: Continued attacks
The suspect continued moving through the third, second and first floors, shooting at groups of teachers and students.
He also encountered a janitor but did not harm him because he was not a teacher. The suspect then headed to Building 4 while reloading his gun.
Stage 4: Confrontation with police
At 10.08am, officers attempted to talk with the suspect and ordered him to stop shooting.
At 10.10am, the suspect shot himself dead.
The suspect's parents spoke to reporters on Sunday, apologising to those affected by the tragedy.
