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.

Believing it to be a fake bomb, officers took it inside and worked on it, investigators said. But it exploded, killing State Police Senior Trooper William Hakim and Woodburn Capt. Tom Tennant. Woodburn Police Chief Scott Russell was critically injured.

The elder Turnidge has had financial problems including debt left from payments of thousands of dollars for various claims over the years. He had struggling businesses in Oregon and Nevada, where records show he had an excavating business and lived on a farm from the late 1990s to 2006, when he and his wife returned to Oregon.

Members of a family that owned the Nevada farm had a legal dispute with Bruce Turnidge over ownership of a mobile home.

"Bruce was a construction type guy ... he could do a lot of things," said rancher John Falen this week. "But he was never successful at any business."

Falen's son, Frank Falen, a Wyoming attorney, told The Associated Press that Turnidge asserted ownership of a mobile home that was on the family's land. But, Frank Falen said, Turnidge dropped the claim after the Falens gave him $2,000 or $3,000 to cover the cost of a stove and refrigerator Turnidge had purchased.

The next hearing in the Turnidges' case is set for Jan. 6.