A father and son accused of killing two Oregon law enforcement officers in a bank bombing pleaded not guilty Friday to charges that could lead to the death penalty if they are convicted.

Bruce Turnidge, 57, and his 32-year-old son, Joshua, appeared in back-to-back hearings Friday on multiple counts of aggravated murder. A grand jury indictment released Friday alleges that the bomb that exploded Dec. 12 was part of a bank-robbery attempt, but it does not say how the men allegedly intended to get the money.

Authorities had not previously specified a motive for the bombing in Woodburn, about 30 miles south of Portland. Prosecutors have refused to talk about details outside the courtroom.

"As for getting into the specifics, I can't say anything more than what's alleged in the indictment," said Matt Kemmy, a deputy district attorney who's been handling the case.

Investigators previously disclosed that a caller to a Wells Fargo office in Woodburn on Dec. 12 made a threat and said that further instructions would come from a cell phone near a garbage can. Officers said they determined that the phone was not part of an explosive device.

Later in the day, investigators turned their attention to the West Coast Bank office next door, and a green metal box was found outside.