Davao City mayor Rodrigo Duterte holds a sub-machinegun as admirers take a selfie with him during the 23rd Defense and Sporting Arms Show at SM Megamall on Wednesday. In his speech, Duterte again espoused his idea of governance despite being non-committal to running for the presidency. Fernando Sepe, Jr., ABS-CBN News

MANILA - Former Bayan Muna party-list representative Teodoro 'Teddy' Casiño said there is no smoking gun linking Davao City Mayor and incoming President Rodrigo Duterte to the alleged death squad killings in the city.

Speaking to radio DZMM on Friday, Casiño said extra-judicial killings in Davao City are a "ticklish issue" between rights advocates and the Davao mayor, who has led the city for over two decades.

"Apparently, hinahayaan at ini-encourage pa nga ito ni Mayor Duterte and that has always been an issue of disagreement sa amin, at mismo yung mga kasamahan namin sa Davao City ang magsasabi na yan ay ongoing ticklish issue. Hindi pinagkakasunduan yan ni Mayor Digong at sa amin," he said.

"But of course, wala naman talagang lumalabas pang sabihin na nating 'smoking gun' or proof na siya mismo ang nag-o-order ng mga ito. As I said, it is an issue that we are engaged in with Mayor Duterte, and we hope na kapag siya ang maging pangulo ay hindi maganap sa buong Pilipinas yung mga naganap na extrajudicial killings sa Davao," he added.

"This will always be an issue na pagtatalunan namin siguro ni Mayor Duterte when he becomes president."

Casiño said human rights groups are opposed to extra-judicial killings, saying the president should not encourage such activities "because it will be hard to control when it starts happening nationwide."

"It will be the end of due process. We have always reminded Mayor Duterte about these dangers," he said.

READ: Don't be a dictator, priest tells Duterte

Human rights groups have documented at least 1,400 killings in Davao that they allege had been carried out by death squads since 1998. Most of those murdered were drug users, petty criminals and street children.

A four-year probe into such killings by the National Bureau of Investigation has not led to a single prosecution.

READ: Death squads under Duterte presidency: Quo vadis?

Duterte earlier vowed that if elected president, he would kill 100,000 criminals and dump so many in Manila Bay that the "fish will grow fat" from feeding on them.

READ: Kill the criminals! Duterte's vote-winning vow in Philippines

He also said he will seek to restore the death penalty in the Philippines, warning he will hang the most heinous criminals twice: once to kill them, then again to "completely sever the head from the body".

This week, Duterte told journalists not to believe his "preposterous statements."