Jimmy Lee Dykes, 65, wanted to go on live television to make a statement, said District Attorney Kirke Adams, but he never told negotiators exactly what he wanted to say.
"He never let on what his message was going to be," Adams said in an interview with The Associated Press.
Dykes never clearly stated a motive for his actions, Adams said, but the man was due in court the day after the standoff began for a hearing on a menacing case filed in December after he allegedly fired a gun at neighbors.
Still, Adams said, officials aren't sure even the menacing charge was the reason for the standoff.
"We had no contact with him since his arrest before this happened," Adams said.
Neighbors said Dykes often went on rants against the government. A longtime acquaintance, Roger Arnold, said Dykes believed the government used electronic shocks to control the racing dogs at a track he frequented.
Authorities said Dykes fatally shot bus driver Charles Albert Poland Jr., 66, and grabbed a boy identified only as Ethan off the bus, retreating to a handmade underground bunker where he held the child for six days.
Dykes died of what the coroner said were multiple gunshot wounds during an exchange of gunfire with FBI hostage rescuers on Monday.
Authorities still on the scene of the standoff Friday were digging around Dykes' rural bunker, located at the edge of a farm field. Aerial photos taken by The Associated Press showed workers had partially excavated the small shelter, which neighbors said was made of plywood and lumber lined with masonry blocks.
The FBI said after the raid that Dykes had planted one explosive artifact in a ventilation pipe used by negotiators to communicate with him in his bunker.
Concrete-block steps led up to the shelter's hatch-type door, which was open. Authorities also were digging around the opening of a steel shipping container that neighbors said Dykes used for storage on his 1.5-acre tract near Midland City.
An FBI spokesman said workers had to stabilize the walls of the bunker for the safety of agents working to gather evidence and clean out its contents.
AP photographer Dave Martin contributed to this report.