Superior Court Judge David Perez refused Thursday to give jurors the option of finding Markhasev guilty of second-degree murder, a lesser offense that carries a maximum term of 30 years to life with a chance of parole.
Markhasev, 19, is charged with killing the son of entertainer Bill Cosby during a robbery - a felony murder count that makes him eligible for a sentence of life without the possibility of parole if he is convicted.
The defendant is accused of shooting Cosby, 27, as he changed a flat tire along a dark road early on Jan. 16, 1997.
CBS News Correspondent Sandra Hughes reports that the stakes are high in this high-profile case. Legal experts say the outcome of the trial, once thought sure to yield a conviction, is a toss-up.
An acquittal could be an embarrassment for prosecutors, who already have racked up losses in the O.J. Simpson and Rodney King trials.
Final arguments are scheduled to begin Monday. The defense rested its case Thursday, with public defender Henry Hall arguing that evidence of attempted robbery to support a felony murder charge "is so thin as to be transparent."
The defense has argued that the deck was stacked against Markhasev because police were unwilling to consider other suspects.
"We have no attempt to take property and no property was taken from Ennis Cosby," Hall said, a day after a police offer testified that Cosby's killer didn't bother to steal the Rolex watch he was wearing, a bracelet, and $850.
Prosecutor Anne Ingalls said the defendant's jailhouse letters, including a reference to "a robbery gone bad," offers "a great deal of support for attempted robbery."
"So our felony murder theory is safe," she said.
Hall argued that jurors "should not be put in a positionÂ…to choose either all or nothing."
Perez rejected the second-degree murder instruction, saying, "I think the evidence supports the people's theory of the case. Either the jury accepts it or it doesn't."
Closing arguments are expected Monday.
Before the defense rested, Markhasev's mother testified her son had an alibi that he was helping her move at the time of the killing.
But Vicktoria Markhasev's story was quickly shown to be in conflict with the account she gave police shortly after her son's arrest - that he had gone to a party the night of the killing and never came home until the next day.
Markhasev was arrested two months after the killing on a tip from an acquaintance who called the National Enquirer in hops of collecting a $100,000 reward.
The defense spent less than two days presenting 13 witnesses, who attacked the police investigation and suggested greedy witnesses made up stories to get money from tabloids. They said Markhasev's friend, Eli Zakaria, killed Cosby.
The prosecution called 19 witnesses over five days but was stymied at points by the refusal of felons to turn state's evidence for fear they would face retaliation behind bars.
Michael Chang, who helped lead police to Markhasev, and Zakaria, who was with him the night of the killings, refused to say a word. Chang was held in contempt; Zakaria was never called to testify but was shown to jurors so they could see if he matched a witness description of the killer.
The most important prosecution witness was Christopher So, the reward seeker who told of going with Markhasev and Chang to hunt for the discarded weapon.
So, who has a criminal record, said he was jeopardizing his life by testifying, and "If I'm ever incarcerated again, I can kiss my life goodbye."