Cordillera Autonomous Region (CAR) police spokesman Superintendent Joseph Adnol said the gunman identified as 18-year-old Joel Melod has sent surrender feelers.
Adnol said charges of murder will be filed against Melod, a member of the Magnao-Lubuagan tribe.
Melod was identified by witnesses through a police artist sketch.
Witnesses claimed Melod climbed up the stage before shooting Diasen, who was in the middle of a campaign speech in Barangay Magnao in Tabuk on Black Saturday.
Authorities earlier theorized that the assassination was part of a tribal vendetta and not election-related.
Investigators noted the killing of Diasen was similar to the assassination of former judge Milnar Lammawin two years ago, which they believed also stemmed from tribal disputes.
Lammawin was killed by gunmen coming from the Mountain Province, where Diasen had his roots.
Meanwhile, the opposition Pwersa ng Masang Pilipino (PMP) urged Diasen supporters to stay calm and allow authorities to bring the killers to justice.
"While we recognize that his death is another tragic blow to Philippine democracy, justice will not be served by resorting to more violence," said Pablo Casimina, PMP director for political affairs.
Casimina claimed Diasen is the PMP’s gubernatorial bet for the province, not of the administration Lakas-Christian Muslim Democrats (Lakas-CMD) as earlier reported.
"While it is true that Vice Governor Diasen originally planned to run as congressman under Lakas-CMD, he changed his mind at the last minute and instead filed again as candidate for governor under the opposition," Casimina said.
Diasen’s nomination was signed by PMP secretary general and former agrarian reform secretary Horacio "Boy" Morales, added Casimira.
In 2004, Diasen, who was Tabuk town mayor for three consecutive terms since 1986, also ran for his current post under the Koalisyon ng Nagkakaisang Pilipino (KNP), the opposition coalition that campaigned for the presidency of the late Fernando Poe Jr.