The father of the deceased shooting suspect in Thailand’s Debsirin Nonthaburi school issued an apology on Sunday to the victims and their families affected by the teen’s deadly rampage.

A 14-year-old boy opened fire at his school on Friday, killing six people on the premises before turning the gun on himself.

Hours earlier, he had allegedly shot dead his grandparents at home before coming to the school in Nonthaburi province, just outside Bangkok, police officials said.

At least 14 people remain hospitalised, of whom seven remain in critical condition Saturday, said Thailand’s health ministry. It said most of those wounded were between 12 and 14 years old.

More than 20 people were injured in Thailand's worst mass shooting since 2022, which has reignited debate over firearms control in Southeast Asia's largest gun-owning nation.

The deceased shooter’s father emerged from the Plai Bang police station on Sunday morning after being questioned by the investigators.

He was seen swarmed by a crowd of journalists and camerapersons on his way to the car.

He issued his first remarks since the incident and apologised in Thai. “I’m sorry,” he said, reported ABC News.

“I am deeply sorry to all the families,” he said in a brief statement.

Authorities have continued their investigations into the rampage as police officials on Sunday said the boy had used social media to learn how to use a firearm and watched violent content online.

In a similar incident, police said, the 14-year-old had an air gun confiscated by a teacher last year.

"According to witness interviews, the perpetrator had begun showing interest in studying the use of firearms for some time, about one to two years," said Atthapol Anusit, deputy commissioner of the Provincial Police Region.

The official said police so far had interviewed 17 witnesses. A knife had also been found in the boy’s bag and other evidence found at the crime scenes included a firearm, spent cartridge cases and a personal computer.

"After examining the PC, it was found that the child had watched social media content involving violence," the deputy commissioner said, but added there was no evidence yet that he had practiced shooting.

Mr Atthapol said that last year a teacher had confiscated a BB gun, a type of air gun, that the boy took to school.

The boy had lived with his grandparents for 12 years after his parents had separated, he said.

Authorities in Thailand are now facing a major challenge of tightening access and control of guns among its population.

The ministry of education said it would develop new safety protocols over the next three months, including mental health screenings and a system to refer at-risk people to specialists, drills for emergency response plans, a drive to stamp out bullying and better detection to stop items from being taken into schools.