Mayor Eleuterio Mabanag, 58, was hit in the left side of his abdomen, and his bodyguard, SPO1 Nestor San Juan, in the head. Responding lawmen found Mabanags body in an irrigation canal in Barangay Margaay.
Their companion, Rogelio de Leon, was critically wounded.
As a result of the killing, Philippine National Police chief Director General Edgar Aglipay sacked Senior Superintendent Rolando Rabara as provincial police director. Aglipay flew here upon learning about the incident.
Mabanag had reportedly been receiving death threats. Despite this, he still went on with his early morning jogging but would change venues from time to time.
Mabanag, who belonged to Lakas-CMD, defeated Erik Ong in the tightly contested May 10 mayoral polls. He was elected vice mayor in 2001 but later took over the townhall when then Mayor Juanito Albano died due to an illness.
Police said the three gunmen, which struck at about 4 a.m., appeared to be hired killers armed with assault rifles. But they would not identify any suspects.
The killing may have been sparked by a political or personal grudge, police chief Inspector Leland Benigno said.
Senior Superintendent Romeo Espiritu, regional police director for operations, said probers were looking into all angles in the killing.
Last year, about 100 residents demanded Mabanags resignation for alleged abuses, including indiscriminate firing of a gun, in a protest led by his political opponents.
Gov. Ferdinand "Bongbong" Marcos Jr. ordered the police to fast-track their investigation. Vice Gov. Wendel Chua also condemned the "senseless killing."
Marcos earlier said that guns-for-fire from another province were responsible for the killings in Ilocos Norte.
Two of the victims of unsolved killings in the province were Batac Regional Trial Court Judge Ariston Rubio, who was waylaid on his way to court in 2001, and Roger Mariano, commentator of radio station dzJC who was gunned down in San Nicolas town last July 31. With Artemio Dumlao