Malvo's lawyers had hoped to put Muhammad on the stand this week. But defense attorney Craig Cooley said Monday that Muhammad's lawyers informed him in writing that Muhammad would invoke his Fifth Amendment right against self-incrimination.
Cooley asked for Muhammad to appear in court so the jury can see him, but Circuit Judge Jane Marum Roush said she saw no need, though she urged the prosecution and defense to try to find a solution.
"Transporting (prisoners) around the state willy-nilly should be done sparingly" because of security risks, she said.
Malvo's lawyers are presenting an insanity defense, claiming the 42-year-old Muhammad brainwashed their teenage client and molded him into a killer.
Muhammad's trial ended last week with a jury recommending the death sentence, but he still faces prosecution in other states. The pair are charged or suspected in the killing of 10 people and the wounding of six in the Washington-area sniper spree, in addition to shootings in Washington state, Arizona, Georgia, Alabama and Louisiana.
CBS News legal analyst Andrew Cohen had been skeptical that Muhammad would actually appear.
"The only way Muhammad takes the stand and testifies in favor of Malvo is if Muhammad himself makes a decision that he is doomed anyway following his conviction and death sentence recommendation, so he might as well help out the young man," Cohen said.
On Monday, Malvo's lawyers played an audiotape of the event they say triggered last year's sniper rampage: the 2001 custody hearing in which Muhammad lost his children. They also called his ex-wife Mildred to the stand.
Malvo's lawyers contend the killing spree was part of a plan by Muhammad to regain custody of the children. They say he planned to kill his ex-wife and make it look as if she were the random victim of a sniper.
Mildred Muhammad testified Monday that on Oct. 11, 2002, in the midst of the sniper spree, she saw a suspicious car in her neighborhood and called 911 to report it. She said it was a dark Chevrolet Caprice or Impala with New Jersey tags, with two people inside whose faces were obscured. That description matches the 1990 dark blue Caprice owned by Muhammad, which prosecutors say served as his "killing platform."
Mildred Muhammad was not allowed to testify about a threat Muhammad allegedly made to her in 2000, in which he purportedly said, "You have become my enemy, and as my enemy I will kill you." The judge ruled that the statement was hearsay and therefore inadmissible.
Prosecutors had also objected to the tape of the custody hearing, calling the theory that the killings were part of a plot to target Mildred Muhammad "nonsense."
"It is absolutely a red herring in this case," Fairfax County Commonwealth's Attorney Robert F. Horan Jr. said.
Muhammad speaks very little during the Sept. 4, 2001, hearing in Tacoma, Wash., in which a judge grants immediate custody of the three children to Muhammad's ex-wife, Mildred Muhammad. Authorities had taken the children from him and placed them in protective custody five days earlier.
Muhammad's prosecutors were prevented from making the argument about Mildred Muhammad because the judge ruled there was a lack of evidence to support it.
Malvo, in statements to police, said the killings were primarily motivated by a scheme to extort $10 million from the government.