SALEM — A Marion County man with a background of financial troubles and his son were bent on robbing a bank in Woodburn when a bomb exploded and killed two Oregon law enforcement officers, says a grand jury indictment released Friday.

The indictment was the first time authorities publicly pinpointed a motive for the Dec. 12 bombing.

Bruce and Joshua Turnidge pleaded not guilty Friday to multiple counts of aggravated murder, which could carry the death penalty if prosecutors seek it.

The indictment says the two were committing robbery and the deaths of a State Police bomb technician and a Woodburn officer were caused in the course of that crime.

The indictment, however, did not describe the mechanics of a robbery, such as a demand for money. Prosecutors have refused to talk about details outside the courtroom, as they did Friday.

“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 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 next-door West Coast Bank office, 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.

At their hearings on Friday, the Turnidges said little.

Bruce Turnidge, 57, responded, “I understand,” when Judge Joseph Ochoa told him about a procedural hearing scheduled for Jan. 6. He stood silent as his court-appointed attorney, John Storkel, entered a not guilty plea on his behalf.

Joshua Turnidge, 32, entered his own plea: “Not guilty, your honor.”

Ochoa said the two suspects are not expected to attend the Jan. 6 hearing.