Isham and Deputy District Attorney Courtland Geyer declined to release the suspect's name, saying it would jeopardize the integrity of the investigation and the safety of officers still working the case.
"We do believe the person responsible for the bombing is in custody," said Geyer, sidestepping the question of whether the alleged bomber had accomplices.
The arrest was made in Salem, roughly 20 miles south of where the explosion occurred - the small city of Woodburn. The name of the suspect and the list of charges will be released by late Monday afternoon, Geyer said.
It's unknown if a possible motive will also be revealed.
Geyer wouldn't disclose if the man in the photos, apparently taken with a security camera, is the same person in custody. The man in the pictures has an average build and appears to be in his 30s. He has a beard and dark hair.
Geyer also wouldn't say if a tip led to the break in the case or provide the exact location of the arrest.
Earlier Sunday, Isham said cell phones and items that might have been used to make the bomb were bought in the central Oregon city of Bend last month. Authorities would not elaborate on how cell phones might have been used. Bombers sometimes use cell phone signals to remotely detonate explosives.
Geyer said revealing anything about the "manner and build" of the bomb would hurt the investigation.
Isham said the man in the surveillance photos may be skilled in welding and electronics, suggesting the device might have been somewhat sophisticated.
The manager of a Woodburn branch of West Coast Bank found the device Friday after a call about a bomb threat to a nearby Wells Fargo bank branch that turned up a harmless device. The bomb was found outside, but the officers took the bomb into the bank, where it exploded.
Lt. Sheila Lorance of the Marion County Sheriff's Office, the lead agency in the case, did not have an explanation Sunday for why the officers took the bomb into the bank.
Woodburn Police Chief Scott Russell remained in stable but critical condition Sunday 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.
Robert Sznewajs, the CEO of West Coast Bank, said Sunday that the bank planned to establish a fund for the families of the law enforcement officers.