"We do believe the person responsible for the bombing is in custody," Deputy District Attorney Courtland Geyer said late Sunday.
The suspect was arrested in Salem, located just south of Woodburn, the small city south of Portland where Friday's bombing occurred.
Officers made the arrest shortly after Sheriff Russ Isham of Marion County released surveillance photos of a "person of interest." Geyer would not say if a tip led to the arrest.
He also wouldn't disclose if the man in the photos, apparently taken with a security camera, is the same person in custody.
Isham would not release the precise location where the arrest took place, and said the suspect's name likely wouldn't be released until Monday afternoon.
"I'm really proud of those who tirelessly worked to get us to this point and am humbled by the community's support," Isham said. "We know there is still a lot of hard work ahead of us, but this development will help bring relief to the local community and the officer's families."
Earlier Sunday, Isham said cell phones and items that might have been used to make the Woodburn 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 often use cell phone signals to remotely detonate explosives.
Geyer said revealing anything about the "manner and build" of the bomb would hurt the investigation.
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.
Woodburn Police Chief Scott Russell remained in 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.