Authorities searched for clues Sunday as a police chief lay in critical condition after a bomb blast at a bank that also killed two officers.

The bomb exploded Friday at a West Coast Bank branch office in Woodburn, a city in the agricultural Willamette Valley south of Portland.

Lt. Sheila Lorance of the Marion County sheriff's office, the lead agency in the case, said investigators were being thorough as they searched for a motive or suspect.

She did not provide any new details, including an explanation for why the officers who ended up dead or wounded took the bomb into the bank after it was found outside.

"There has been a lot of speculation about that, but that has yet to be determined," said Lt. Gregg Hastings, spokesman for the Oregon State Police, which is assisting in the investigation along with the FBI and the federal Bureau of Alcohol, Tobacco, Firearms and Explosives.

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.

A $35,000 reward has been offered for any tips in the bombing, and police asked the public for help seeking the bomber.

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.

Sznewajs said all records and money kept at the branch were secure, and it would reopen as soon as possible.