Even as an unprecedented 9,000 prospective jurors were summoned for questioning, both sides in the mass murder trial are worried about letting too many potential jurors go. Prosecutors have asked the judge not to reveal why he releases jurors, for fear of handing out a road map for others trying to avoid serving. [...] defense attorney Daniel King warned the judge who was listing off the reasons to let people go: "You have to consider the fact that people may not want to sit on this jury." Prosecutors in the Holmes case rejected a plea offer from defense lawyers, pushing the case toward a grueling trial that will replay the massacre. Jurors will be shown graphic photos from the theater, where police say Holmes slipped in through in a back door wearing a gas mask and body armor, threw gas canisters into the audience and opened fire during a midnight showing of a new Batman movie on July 20, 2012. Research has shown jurors in death penalty cases have suffered nightmares, flashbacks and symptoms similar to post-traumatic stress disorder, said James Acker, a researcher on death penalty juries at the State University of New York in Albany. Federal prosecutors in Boston are confronting a similar dilemma as they try to find people to serve as jurors for the murder trial of accused Boston marathon bomber Dzhokhar Tsarnaev, another death penalty case. [...] many notices went out that witnesses to the attack received them, as did relatives of staffers in the local district attorney's office.