* Former neuroscience graduate student killed 12 in attack
* Defense argues Holmes right to fair trial jeopardized
By Keith Coffman
DENVER, Oct 11 (Reuters) - Prosecutors and attorneys defending accused Colorado theater gunman James Holmes return to court on Thursday to spar over leaks to the news media, and the status of a hearing that could provide details of the massacre.
Arapahoe County District Court Judge William Sylvester will hear the two issues in the latest of a series of pre-trial proceedings in the sensational murder case.
Holmes, a 24-year-old former neuroscience graduate student, is accused of opening fire inside a suburban Denver movie theater during a midnight screening of the Batman movie "The Dark Knight Rises' on July 20.
The rampage, one of the worst outbursts of U.S. gun violence in recent years, killed 12 people and wounded 58 others.
Public defenders said in court filings that Holmes' right to a fair trial was jeopardized when someone in law enforcement leaked details of a package that Holmes sent to a psychiatrist, in violation of a gag order imposed by Sylvester.
The parcel purportedly contains a notebook detailing plans for the theater rampage, according to a Fox News report.
Holmes' lawyers said they have received 16,000 pages of documents from prosecutors that they say support "the defense's concerns that the government was responsible for leaking information" about the package to the news media.
They are asking the judge to impose sanctions on prosecutors for the disclosures.
Prosecutors said they were unable to respond to the "vague allegations" made by the defense, noting the motion does not identify the specific information that public defenders are complaining about, or if the media reports were even true.
Sylvester will also hear from lawyers on Thursday on whether a preliminary hearing scheduled for next month will be held on time, delayed, or if Holmes will waive his right to the hearing altogether.
If the preliminary hearing is held, prosecutors will lay out details of the case so the judge can rule if there is enough evidence against Holmes to bind him over for trial.
Prosecutors have depicted Holmes as a young man whose once promising academic career was in tatters. He failed graduate school oral board exams in June, and one of his professors suggested he may not have been a good fit for his competitive Ph.D. program.
In open court and in motions filed in the case, prosecutors accuse Holmes of amassing an arsenal of weapons as part of "a detailed and complex plan" to commit mass murder.
On the night of the rampage Holmes bought a ticket to the movie then slipped outside, prosecutors said, armed himself and returned to the theater, spraying moviegoers with gunfire.
Holmes' attorney, Daniel King, who analysts have said appears to be laying the groundwork for a possible insanity defense, has said his client suffers from an unspecified mental illness and had tried to get help before the shooting. (Editing by Cynthia Johnston and Todd Eastham)