Residents discovered the bruised body of high-school student Afif Maulana in a river close to a village in Padang, in West Sumatra on June 9. 

An initial report by the Padang Legal Aid Institute found that police had beaten Afif, and burned, beaten and electrocuted eight other students who were with him at the time. 

Police have denied the allegations, but Komnas HAM said on Tuesday it was opening an investigation into the incident.

"Afif's death was unnatural and we suspect there were unlawful actions by the police," Hari Kurniawan, a commissioner at Komnas HAM told reporters. 

Police have said they were acting to separate students who appeared to be poised to have a brawl. 

"The school fight had not even started so if they had wanted to disperse them, they should have done it in an humane manner," said Putu Elvina, a second Komnas HAM commissioner.

Komnas HAM has deployed investigators to West Sumatra to gather evidence. It is unclear when they will present their findings. 

Speaking to local media, West Sumatra police chief Insp. Gen. Suharyono denied that any of his officers were involved in the death of Afif or the mistreatment of other students, saying there was no evidence from witnesses.

The West Sumatra police said they have questioned 39 police officers so far, and they have not received reports of abuse.