PORTLAND, Ore. - Cell phones might have been used in a bomb blast that killed two officers who had carried the device into an Oregon bank, investigators said yesterday as they released surveillance photos of a "person of interest."

Marion County Sheriff Russ Isham said the man in the photos, who has not been identified, may have experience in welding and electronics.

The photos, apparently taken with a security camera, show a man with an average build who appears to be in his 30s. He has a beard and dark hair.

The Sheriff's Office declined to say where the photos were taken but said someone involved in the bombing might have been in the central Oregon city of Bend last month.

Cell phones and items that might have been used to make the bomb were bought in Bend last month, Isham said. The man might have taken actions "in furtherance of his plan" on Thursday and Friday in Salem and Woodburn.

Authorities would not elaborate on how cell phones might have been used. Bombers often use cell phone signals to remotely detonate explosives.

Lt. Sheila Lorance of the Marion County Sheriff's Office, the lead agency in the case, did not have an explanation yesterday for why the officers who ended up dead or wounded took the bomb into the bank after it was found outside.

A West Coast Bank branch manager found the device Friday after a call about a bomb threat to a nearby Wells Fargo bank branch that turned up a harmless device.

Isham asked the public to help identify the man in the photos. A $35,000 reward has been offered for tips.

"We remind the public of the danger he poses to our community, not only to the potential intended victims, but to others who may know him or are exposed to his explosives," Isham said.

Woodburn Police Chief Scott Russell remained in critical condition yesterday at a Portland hospital as a result of the blast that killed Woodburn police Capt. Tom Tennant and Oregon State Police Senior Trooper William Hakim. *