MANILA (Reuters) - Two motorcycle-borne gunmen on Wednesday killed the vice mayor of a southern Philippine town, the fourth local official assassinated in a week, including one President Rodrigo Duterte said may have had links to illegal drugs.

The attack took to 16 the number of mayors and vice mayors killed since Duterte unleashed his deadly anti-narcotics campaign when he came to power in 2016, media said.

“We’re still investigating the motive for the killing,” said Allan Nazaro, police chief of Zamboanga City, after Wednesday’s attack on Al Rashid Mohammad Ali, vice mayor of the town of Sapa-sapa in the southernmost province of Tawi-Tawi.

A relative was driving Ali in his vehicle when the gunmen opened fire, he added.

Three mayors, who were on Duterte’s watchlist of local officials suspected to have drug links, had been killed in two years. Ali is the second vice mayor to be assassinated.

Last week, Mayor Antonio Halili of Tanauan City in Batangas, south of Manila, was hit by a bullet in the chest during a flag-raising ceremony.

Halili had gained prominence in 2016 for parading drug suspects in the streets, but Duterte had put him on a watchlist of regional leaders.

Duterte said the mayor may have had some drug involvement, calling his campaign to parade suspects was a ploy to convince police he was not engaged in the illegal drug trade. Halili’s daughter has denied that her father had such links.

On Tuesday, gunmen on a motorcycle killed Mayor Ferdinand Bote of General Tinio town in Nueva Ecija province, but police said he was not linked to the drug trade.

Alexander Lubigan of Trece Martires, also south of Manila, was also killed in an ambush on Saturday.