The Philippine National Police should initiate investigations on summary killings linked to drugs, former PNP chief and Senator Panfilo Lacson said on Monday.

Senator Panfilo Lacson told ANC's Headstart the PNP should solve at least some of the 170 summary killings of drug suspects which are reportedly perpetrated by vigilante groups.

"I think the police should initiate investigating the summary killings perpetrated by vigilante groups because 170 unresolved or unsolved murder cases in a month is just too large a number," he said.

Lacson, who served as director general of the national police force from 1999 to 2001, said as Chief PNP, he was "already alarmed" at 10 unresolved murder cases in his agency.

"We’re counting 170, and 173 as of this morning, and I have yet to hear a pronouncement or announcement from the PNP that they are investigating these summary killings," he said.

However, he maintained that a Senate inquiry on these operations is not needed, as investigations for these cases should be done under proper government agencies like the Commission on Human Rights, the Department of Justice, or the Internal Affairs Service under the Department of the Interior and Local Government.

"To drag down the whole leadership and be investigated on account of these alleged summary killings, would or could deter the police from proceeding further. We have a good momentum in our fight against illegal drugs," he said.

Meanwhile, Lacson also lamented that President Rodrigo Duterte's team lacked research on the photo of Jennilyn Olayres cradling her partner, pedicab driver Michael Siaron, after he was shot dead.

The photo published by broadsheet Philippine Daily Inquirer was captioned 'La Pieta' because of its likeness to the sculpture by Michelangelo Buonarroti, depicting Jesus on the lap of his mother Mary after the Crucifixion.

President Duterte branded the photos "melodramatic" in his first State of the Nation Address (SONA) last July 25, and warned drug users and pushers of the consequences of their actions.

READ: Behind 'La Pieta': Widow says her husband a 'victim of drugs'

But Lacson noted, Siaron was killed by motorcycle-riding men who remain at large and unidentified, not during a police operation.

"The president, when he announced that in his SONA, could have just ordered the PNP to investigate and find the killers, and bring them to justice," he said.

"Mukhang hindi na-research nang husto; siguro ang mindset ni Presidente at that time when he was making the announcement, and he was criticizing the broadsheet, was that the police was responsible in killing Michael Siaron—which is not the case," he said.