MANILA - Human rights advocates on Tuesday called for the reopening of the investigation against the Davao Death Squad (DDS), an alleged vigilante group being linked to presumptive president Rodrigo Duterte.

The appeal of Human Rights Watch (HRW) came after the Department of Justice (DOJ) admitted that its key witness has disappeared.

In a statement, HRW Deputy Director for Asia Phelim Kine warned that the justice department's supposed lack of zeal in pursuing the issue clearly "shows a disturbing failure to address the serious problem of government-linked killings across the Philippines."

The HRW reported that based on its investigations in 2009 and 2014, children as young as 14 were among the hundreds murdered by death squads in Davao City, Tagum City and elsewhere in the Philippines.

They claimed public officials and police were directly involved in those crimes.

The advocacy group said the government is duty-bound to prosecute individuals behind the DDS and other vigilante groups.

"The government has obligations under Philippine and international law to bring justice those responsible for the [DDS], the Tagum Death Squad, and similar criminal operations," Kine said.

Outgoing justice secretary Emmanuel Caparas has said the DOJ's investigation could not proceed unless their sole witness resurfaces.

Former justice secretary Leila De Lima, who initiated the probe into the death squads, said Sunday the witness clearly feared Duterte.

"From all indications, the decision of the witness to leave (the protection program) is definitely in reaction to Duterte's (likely)victory," De Lima said.

Duterte earlier vowed that if he is 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.

PH halts probe into suspected death squads