The decision by Circuit Judge Jane Marum Roush allows prosecutors to use Malvo's own words against him at his trial in the Oct. 14 killing of FBI analyst Linda Franklin. Authorities said he confessed to two of the fatal shootings, including Franklin's.
The trial is set to begin Nov. 10.
"Having considered the totality of the circumstances surrounding the police interrogation ... I conclude that his statement was made voluntarily," Roush wrote.
Roush tossed out only some incidental statements made by Malvo before he signed a document with an "X" waiving his right to remain silent and right to a lawyer. Police had characterized that prior information as chitchat, although it included some discussion of Malvo's relationship to fellow suspect John Allen Muhammad, in which Malvo said he learned "everything" from his friend and said he occasionally gave orders to Muhammad.
The most damning part of Malvo's statement, though, can be used by prosecutors. In the statement, authorities have said, Malvo laughed about some of the shootings and chuckled as he recalled a boy's reaction to a missed shot, recalling that the boy swatted at his face as though a bee buzzed by him.
"The judge bought virtually every prosecution argument in reaching her conclusion and specifically ruled that police and prosecutors, and federal and state authorities, did not conspire together to confuse Malvo in order to question him without counsel," says CBS News Legal Analyst Andrew Cohen. "It's a ruling that certainly will be part of an appeal if Malvo is convicted.
"This is a crushing defeat for the defense and will make it nearly impossible for Malvo's attorneys to gain any sympathy at all from jurors, who now will hear these chilling, soulless statements Malvo allegedly made while describing one of the sniper shootings. It's going to hurt the defense at trial and also during a sentencing phase if there is one."
Malvo's attorneys did not immediately return a phone call seeking comment. They cannot appeal the ruling until after trial, in the appellate courts.
Malvo's lawyers had argued that police and prosecutors essentially conspired to keep Malvo away from his lawyers on Nov. 7, when federal charges against Malvo were dropped and he was transferred to Virginia custody. He had been in Maryland.
Malvo's federally appointed lawyers on that day testified that they scrambled to find their client and prevent him from talking to police, but were unsuccessful.
But Roush wrote that "there is no evidence that Fairfax police or prosecutors colluded with federal authorities to spirit Malvo away to Virginia without the knowledge of his Maryland attorneys."
Roush also ruled that Malvo had no right to counsel on Nov. 7, because the federal charges against him had been dropped and the Virginia charges were not formalized until Nov. 8, when he made his initial court appearance. She also ruled that, even if he had the right to a lawyer, he knowingly waived it during his questioning with police.
Fairfax County homicide Detective June Boyle testified that she asked Malvo four different times if Malvo was certain about wanting to talk without an attorney.
She acknowledged, though, that at the very beginning of the interview, Malvo asked, "Do I get to see my attorneys?" Boyle said she answered yes, and Malvo responded, "Because my lawyer said not to talk to the cops until they get here." Boyle answered that Malvo was now facing some new charges in Virginia and they needed to get some information about him.
Defense lawyers argued that the questioning should have stopped as soon as Malvo asked about his lawyers. But Roush disagreed.
She ruled that Malvo's questions about his lawyer are "at best a request for a clarification of his Miranda rights."
The disallowed portion of the interrogation included questions about Malvo's vegetarianism and eating raisins. The defense said that had amounted to an interrogation because a bag of raisins with Malvo's fingerprints was found at one of the crime scenes.
Malvo, 18, and Muhammad, 42, are accused in at least 20 shootings, including 13 deaths, in Virginia, Maryland, Alabama, Georgia, Louisiana and Washington, D.C. Both could face the death penalty if convicted.
Prosecutors have said the shootings were part of a scheme to extort $10 million from the government.
Prosecutors have other evidence linking Malvo to the crimes, including ballistics, fingerprints and DNA evidence, but prosecutors said before the ruling their case would be much easier to prove if the confession was allowed.