Police were scouring a nearby elevated grassy area, where the gunman may have fired the shot.
Halili became controversial two years ago when he ordered drug suspects to be paraded in public in Tanauan, a small city about 70km south of Manila, in a campaign that was dubbed "walks of shame." The suspects were forced to wear cardboard signs that read "I'm a pusher, don't emulate me" in a campaign that alarmed human rights officials.
Police officials, however, also linked Halili to illegal drugs, an allegation he strongly denied. He said at the time that he would resign and would be willing to be publicly paraded as a drug suspect if police could come up with evidence to support the allegation.
Albayalde said investigators would try to determine if the killing was connected to Halili's anti-drug campaign.
Halili's unusual campaign drew attention at a time of growing alarm over the rising number of killings of drug suspects under President Rodrigo Duterte. Since Duterte took office in 2016, more than 4200 drug suspects had been killed in clashes with police, alarming human rights groups, Western governments and UN rights watchdogs.
Human rights groups have reported much higher death tolls, although Duterte and his officials have questioned the accuracy of those reports. They said the suspects died because they opened fire and sparked gunbattles with authorities although human rights groups have accused police of extrajudicial killings.
Halili's killing came a few weeks after a Catholic priest was shot and killed while preparing to celebrate Mass in a village chapel in northern Nueva Ecija province.
Sen. Panfilo Lacson, a former national police chief, urged the police to impose stricter firearms control in light of the killings.
"The killing of priests, prosecutors, and former and incumbent local officials in broad daylight and in full view of the public may be suggestive of the impunity and brazenness of those responsible for such acts," Lacson said.
"The Philippine National Police should feel challenged, if not taunted," he said. "And they must immediately consider stricter firearms control strategies before similar killings could reach ubiquitous levels."
- AP