Senate President Pro Tempore Jinggoy Estrada believes that his colleague, Senator Ronald "Bato" Dela Rosa’s plan to hold a parallel investigation into the "war on drugs" campaign during the time of former President Rodrigo Duterte could be biased.

Dela Rosa, as the chairman of the Senate Committee on Public Order and Dangerous Drugs, earlier announced that the Senate would conduct its own inquiry into the drug war, with plans to invite Duterte as a resource person.

In an interview on Dos Por Dos on Thursday, Estrada noted that while there is no confirmation yet if Dela Rosa would call for a Senate hearing on the matter, he warned that the investigation might be politically tainted due to Dela Rosa’s alliance with the former President.

"Ang sabi ni Sen. Bato, motu proprio ang hearing pero ang problema, okay lang sa'kin na pakinggan ang panig ni Pang. Duterte pero magkakaproblema, kaalyado nila si FPRRD," Estrada said.

Meanwhile, Estrada suggested that if the hearing pushes through, Dela Rosa should consider assigning someone else to handle the committee, such as a subcommittee or another presiding officer, to ensure impartiality.

"Ginawa sa amin noong araw [tanungin ng kapwa-senador], kinakasuhan kami ng tungkol sa PDAF scam. Hindi ko masasabi kung pwede kay Sen. Bato, base sa rules 'di pwede magsalita laban sa kapwa-senador," Estrada said.

During the House Quad Committee hearing on October 11, former police colonel Royina Garma claimed that Duterte called her in May 2016 to help set up a task force to replicate Davao City's anti-drug campaign nationwide.

Garma also claimed that the drug war had a reward system for individuals killed, with rewards based on whether the person was an alleged street-level pusher or a major drug dealer.

The House Quad Committee – consisting of the committees on dangerous drugs, public order and safety, human rights, and public accounts – has held eight public hearings, where Duterte’s name frequently surfaced in connection to extrajudicial killings linked to the drug war.