Superintendent Fidel Cimatu Jr., provincial police director, said the gunmen are between 30 and 40 years old, with fair complexion and are of medium build.
At the time of the ambush, they were wearing maong pants and colored T-shirts, Cimatu said.
Cimatu lamented the scarcity of witnesses to help authorities solve the ambush-slaying of Chan and his driver, Alex Regacho.
He said he has started asking people to contribute to the reward for information on the killers, to make it more attractive to potential witnesses.
Gov. Deogracias Victor Savellano earlier had set aside P200,000 as cash reward for information that may lead to the identification and arrest of the gunmen.
Some quarters here said the reward may not be big enough to entice eyewitnesses to come forward, considering the risks involved.
"Their testimonies will expose their lives and their families to danger," an observer said.
An investigator told The STAR that two witnesses whom the police earlier contacted appeared to be withholding vital information when they were interrogated.
Meanwhile, former Ilocos Sur Gov. Luis "Chavit" Singson, contacted in Los Angeles, California, told dzVV Bombo Radyo yesterday afternoon, that he would ask for a serious investigation into the Chan-Regacho slaying when he returns home next week.
He said he was busy with some family business but would rush home to attend to the ambush-slaying that has rocked the province.
Chan became provincial auditor in October last year just when Chavit was parting ways with former President Joseph Estrada.