(CNN) -- An Arizona judge agreed Wednesday with defense attorneys that prosecutors in the manslaughter trial of self-help author James Ray improperly withheld evidence, but he decided not to grant a defense motion for a mistrial.
Ray is accused of manslaughter in three deaths at his October 2009 "Spiritual Warrior" retreat in the desert. Kirby Brown, 38, of Westtown, New York, died that night. So did James Shore, 40, of Milwaukee. Volunteer Lizbeth Marie Neuman, 49, of Prior Lake, Minnesota, died nine days after the ceremony. At least 15 others who took part in the sweat lodge ceremony became ill. More than 40 others were uninjured.
Prosecutors argued that the lodge -- made of willow trees and branches, and covered with tarpaulins and blankets -- was heated to a perilously high temperature, causing the participants to suffer dehydration and heatstroke.
Defense attorneys filed a motion Monday accusing prosecutors of intentionally withholding a report from environmental expert Richard Haddow, which suggests that the faulty design of the sweat lodge caused or contributed to the deaths.
"At issue is a report, or an e-mail, drafted by an environmental engineer that states two relevant factors," defense attorney Luis Li told Judge Warren Darrow during Wednesday's hearing. "One, that carbon dioxide, not heat stroke, is a possible cause of death. ... And secondly, that various environmental conditions created by the design of the sweat lodge -- for which Mr. Ray has no responsibility -- could have contributed to those deaths."
Li told Darrow that prosecutors' response to the motion was "aggressively unrepentant."
Defense attorneys contend that a "Brady violation" has taken place. A Brady violation occurs when the failure to disclose material exculpatory evidence to the defense deprives a defendant of a fair trial. Exculpatory means the evidence could be used to establish a defendant's innocence; material means the evidence is relevant and significant.
Prosecutor Bill Hughes said the state doesn't believe that the conduct met the criteria for a Brady violation. "It was not disclosed due to an oversight," Hughes said, adding that the state has turned over thousands of pages of information.
Darrow said he agreed that a Brady violation had occurred but declined to grant a mistrial. The trial is set to continue Thursday.
Haddow's findings were in a report that was e-mailed to lead detective Ross Diskin almost a year ago, on April 29, 2010.
Ray's attorneys first learned about Haddow when he was listed as a potential witness in late October. Since November, they say, they asked prosecutors for Haddow's report four times. They got it last week.
Though the state decided not to call Haddow as a witness at the trial, Arizona's discovery law requires prosecutors to provide the expert's report. There is also an ongoing obligation by prosecutors to turn over all material that could exonerate a defendant.
Because they are receiving Haddow's report several weeks into the trial, and because that testimony could have had a profound impact on their strategy, defense attorneys argue that the judge has no choice but to grant a mistrial.
The sweat lodge ceremony consisted of eight rounds, with each round lasting 10 to 15 minutes. Although they were not prevented from leaving, participants have said they were told to wait until the breaks between rounds.
Ray's attorneys have argued that the deaths were accidental and suggested that exposure to an unknown toxin in the lodge -- perhaps a pesticide, rat poison or something in the type of wood used to heat the rocks -- could have caused the fatalities.
The event participants paid up to $10,000 to seek "new areas of consciousness" at the retreat, according to testimony. Many had attended previous James Ray International seminars.
In Session's Beth Karas and Michael Christian contributed to this report.