Lawyers for Washington sniper suspect Mr Lee Malvo (18) will argue the teenager was not responsible for the killings because he was subject to "indoctrination" when his case opens next month.
Mr Malvo's attorney, Mr Craig Cooley, said the jury will be told that fellow-suspect Mr John Muhammad had influence of such a degree that his client could be considered insane during the three-week shooting spree that terrorised the Washington area a year ago.
Mr Malvo and Mr Muhammad were arrested on October 24th, last year, on charges stemming from the sniper-style killings of 10 people and the wounding of three more in Maryland, Virginia and Washington.
Virginia prosecutor Mr Robert Horan said the mental-health report of experts who had examined Mr Malvo more than a dozen times in the last five months made no reference to insanity.
Mr Muhammad's trial is to begin next Monday in Virginia Beach, Virginia. He has been denied the opportunity to plead insanity because he has refused to undergo a state examination.
Mr Malvo's trial is scheduled to begin on November 10th in Chesapeake, Virginia.
Both trials were moved about 200 miles from the Washington vicinity after attorneys argued fair trials would be impossible in an area where there was saturation coverage of the crimes.