Senior Superintendent James Melad, Cagayan police director, said they have suspects in the ambush of MTC Judge Nathaniel Pattugalan, 59, of Baggao town, but withheld their names while more evidence is being gathered against them.
Melad said the ambush could be job-related.
Aside from the NBI, Melad said the Criminal Investigation and Detection Group has also joined the composite team trying to unmask the ambushers.
Pattugalan, who managed to trade shots with the gunmen, survived the attack with only minor gunshot wounds.
His driver, Antonio De la Cruz, 37, however, was killed.
Pattugalan was on his way to Tuguegarao City from the Baggao town proper where he had presided over a court hearing when he was waylaid along an isolated stretch of the provincial road in Baggao.
At least 15 empty shells from M-14 and M-16 Armalite rifles were found at the ambush site, Melad said.