MANILA - Senate Minority Leader Franklin Drilon on Thursday proposed tapping the Office of the Ombudsman to investigate the controversial shooting of 11th grader Kian Delos Santos as he called out Justice Secretary Vitaliano Aguirre II's 'bias' for the police.

At a Senate hearing on Delos Santos' shooting, Drilon broached the idea of having the Ombudsman investigate the teen's death in a police operation in Caloocan City on Aug. 16 to ensure an impartial probe.

The Ombudsman is primarily mandated to investigate cases of wrongdoing committed by public officials.

"Ang bias ninyo is so obvious. Your bias to protect the policemen is so obvious. I do not see any indication that you are trying to seek a neutral fact here," Drilon told Aguirre at the hearing.

Aguirre earlier said the Delos Santos case has been blown out of proportion, adding that collateral damage may not be avoided in any war.

The teen's shooting has drawn heavy public condemnation as evidence, including security footage and witness statements, suggest that Delos Santos was executed.

"I said this is blown out of proportion because lahat sumakay na dito lalo na 'yung nasa human rights groups, 'yung mga group na wala nang nakita na palaging masama ang ating Pangulong [Rodrigo] Duterte," Aguirre explained at the hearing.

Aguirre again pointed out that none of those who have condemned Delos Santos' killing seemed bothered when a family was massacred in Bulacan.

But Drilon cited the difference between the two cases: "It is the state agents, pulis 'yung pumatay [in Delos Santos' case]. That is the basic difference when you use the forces of government that resulted in the death of a citizen."

But is Aguirre willing to submit to the Ombudsman's authority? He said he would not.

"Because that will put us in bad light. If I agree with you, then it would appear that we are incapable," he said.

Aguirre earlier questioned the credibility of witnesses under the custody of opposition lawmaker Sen. Risa Hontiveros and the Integrated Bar of the Philippines.

• READ: Aguirre: Witnesses in Kian slay may have been 'brainwashed'

The witnesses are set to reveal in an executive session what they know about Delos Santos' death.

Witnesses have said that before the boy was killed, police gave him a gun and told him to run for his life. CCTV footage also showed police dragging Delos Santos to the spot where he was later shot dead.

Autopsy reports also showed that the boy was already down on the ground when shot.

• READ: PNP, PAO agree on autopsy: Kian already down when shot