MANILA — Sen. Ronald "Bato" dela Rosa on Thursday criticized the House inquiry into the anti-drug campaign of former President Rodrigo Duterte, saying "there is no clear statement" in the resolution about it being in aid of legislation.

Congressional hearings or investigations are part of the oversight function of the legislative branch and are usually conducted to help lawmakers see gaps in the law that need to be addressed.

Speaking on ANC's "Headstart," Dela Rosa initially said that he respects that House lawmakers are "free to do their thing."

But he later said in the interview that it was not the legislature's territory to conduct investigations into Duterte's war on drugs, pointing out that it had long been looked into even before former Senator Leila de Lima was detained.

READ: De Lima acquitted on last of three drug cases

He added that the Philippines' criminal justice system is already investigating the drug war deaths during Duterte's time.

"Dapat hindi na 'yan kanilang turf 'yung pag-iimbestiga dahil ang ginagawa nila ngayon is no longer in aid of legislation kundi para mapanagutin kuno ang mga pulis na involved diyan sa war on drugs," Dela Rosa said, referring to the ongoing congressional probe.

(That's not their turf to investigate because what they are doing now is no longer in aid of legislation but supposedly to hold the police involved in the war on drugs accountable.)

Thousands of drug suspects were killed under Duterte's anti-drug campaign that Dela Rosa implemented, which is the subject of an International Criminal Court investigation looking into alleged crimes against humanity.

The House Committee on Human Rights has said it would invite Dela Rosa and Duterte to attend its inquiry.

Dela Rosa said that he would not attend the probe because Senate President Francis Escudero had told him that he does not need to.

In the ANC interview, however, Dela Rosa said he would show up at the hearing if Duterte decides to appear.

"Out of courtesy to my former boss. Nakakahiya naman he will be the one answering all the questions na dapat ako ang magsasagot. Kailangan puntahan ko talaga. I fear the time pag dumating tayo sa punto na ‘yan that I will be breaking the time-honored and institutionally enshrined tradition of both houses ‘yung interparliamentary or inter-chamber courtesy," he said.

(Out of courtesy to my former boss, it would be embarrassing if he has to answer all the questions that I should be answering. I really need to be there. I fear the time when we reach that point where I will be breaking the time-honored and institutionally enshrined tradition of interparliamentary or inter-chamber courtesy between both houses.)

Chambers of Congress generally observe inter-parliamentary courtesy and invite, but do not summon, colleagues to hearings and inquiries.

READ: Kin of drug war victims dare Duterte, Dela Rosa to attend House probe

ACT Teachers party-list Rep. France Castro on Thursday urged Duterte to attend the House probe on his drug war as she expressed doubts that the former president would show up at any court hearing related to the anti-narcotics campaign.

Castro urged Duterte to show courage by facing the families of drug war victims and by accepting the invitation of the House panel.

"Totoo ba na pupunta siya sa mga korte? E samantalang noong nag-file ako ng kaso laban sa kanya, ay hindi naman siya sumipot sa dalawang instance na dapat siya ay sumipot. Nagkung ano-anong dahilan ang kanyang abogado," she said.

(Is it true that he will face the court? Because when I filed a case against him, he did not show up on two occasions when he was supposed to appear. His lawyer came up with all sorts of excuses.)

Castro pointed out that Duterte would have an opportunity to defend his drug war at the House hearing.

"Kung ako sa kanya, maging matapang siyang harapin," Castro said.

(If I were him, he should be brave enough to face it.)

—with a report from Vivienne Gulla, ABS-CBN News