A 13-year-old boy was killed in broad daylight on Monday in a brazen crime caught on camera by onlookers in Phnom Penh’s Prey Sar commune.

The video, which was circulated on social media on Tuesday and shot approximately 10 metres from the murder, shows a man kneeling in a shallow pool of water drowning the boy.

Chim Sitha, Dangkor district police chief, said that the suspect – Var Ratha, 33 – was being questioned by police and would be sent to court today. He said he had not seen the video.

“When we asked him why he beat the boy until he died, he said he was angry with the boy, who used a kamplus [a type of Cambodian toy gun] to shoot him [with a spitball],” he said. “The suspect said he had a ‘master spirit’ in his body that caused him to be angry.”

Sitha said the suspect fled after the crime, but other villagers held him.

Police initially suspected drug abuse played a part in the crime, he said, but a urine test did not turn up any illicit substances.

James McCabe, operations director of the Child Protection Unit, said the murder was believed to have happened at about 3:30pm when the boy was herding cows. He added that Ratha was likely to be charged with “aggravated murder”.

McCabe added that he was not surprised witnesses had not intervened, and noted that the recording of the murder had at least led to the arrest of the offender. “When individuals see such a violent interaction between people, it is quite normal to be reluctant to run into water and confront an offender who is attacking someone,” he said.

Moreover, he said, this could have also had a traumatic effect on the bystanders.

“You can’t be critical . . . of people’s inaction, especially when seeing and being confronted with such a very violent, horrific crime being committed in front of them . . . You can hear the bystanders were yelling, trying to draw his attention . . . but unfortunately with this offender, it had no effect,” he said.

Chhim Sotheara, executive director of the Transcultural Psychosocial Organization, said in an email that when people witnessed traumatic incidents, they would typically react with a “fight or flight” response, but that in some cases experience a “freeze reaction”.

Sotheara added that the response could have roots in prior trauma, and referred to a unique Cambodia trauma response called baksbat, or broken courage. Those who exhibit the response tend to have experienced or witnessed “major” traumatic events, and respond with “submission to defeat, [and dare] not stand up or resist”, he said.

Updated with additional comment: Monday 22 November 2017, 7:01am