A motion was filed to limit the testimony of David Paul Hammer, who is expected to testify for the defense at Nichols' trial on 161 counts of first-degree murder for the April 19, 1995, bombing of the Oklahoma City federal building.
Hammer is expected to testify that bombing coconspirator Timothy McVeigh told him there were other accomplices in the bombing. The conversations took place while he and McVeigh were on federal death row in Terre Haute, Ind.
Nichols' defense attorneys claim that McVeigh had substantial help from others in staging the bombing and that Nichols was set up to take the blame.
In a motion filed Tuesday, Assistant District Attorney Lou Keel said Hammer's testimony cannot be used as to show that McVeigh had help in planning and executing the bombing from members of the Aryan Republican Army.
Hammer's testimony can only be used to challenge the testimony of Michael Fortier, the prosecution's star witness, the motion states. Fortier is expected to testify that McVeigh told him Nichols was involved in the bombing.
To hold otherwise ''would allow any thrill-seeker, enemy of law enforcement or friend of a defendant to poison the evidence heard by a jury with whatever their imagination might provide,'' Keel said.
No hearing is scheduled on the motion.
A 12-member jury was seated last week for Nichols' murder trial. Opening statements are scheduled Monday.
A federal jury in Denver convicted Nichols in December 1997 of conspiracy and the involuntary manslaughter of eight federal agents in the bombing, which killed 168 people. A judge sentenced him to life in prison.
The state trial covers the deaths of the other 160 people who were killed and the fetus of one of the victims.
Prosecutors allege Nichols helped McVeigh assemble the ingredients for the bomb and build it. Hammer claims McVeigh told him Nichols helped store the components for the fertilizer and fuel oil bomb but balked at any deeper involvement in the plot.
Hammer, a former Oklahoma prisoner, is scheduled to be executed June 8 for murdering a prison cellmate.
McVeigh was convicted on federal murder charges for the bombing and was executed in June 2001.