California woman slain by 'mortar-like device': prosecutor El CAJON, California (Reuters) - A California man did not mean to kill his girlfriend when he ignited a makeshift "mortar-like device" and it exploded, sending shrapnel into their trailer home that struck and killed the woman, a prosecutor said on Thursday. Richard Fox has been charged with murder over the woman's death earlier this week, in a case a legal expert said will likely hinge on accusations he acted recklessly. The bizarre and grisly fatality of the 39-year-old Fox's girlfriend, Jeanette Ogara, alarmed residents in the remote border community of Potrero where the couple lived, and left the couple's families in grief. Police initially said the weapon, which was set up outside the home, was a homemade cannon. But San Diego County deputy district attorney C.J. Mody on Thursday called the contraption an "improvised mortar-like device" with a 26-inch metal tube, a welded base and a fuse at the bottom. Police shortly after Fox's arrest said it was too early to tell if Ogara's death from the explosion was accidental. Mody on Thursday clarified the position of authorities. "There is no evidence this was intentional," Mody told reporters outside the courtroom. Fox, Ogara and three adult guests had been drinking before the explosion, Mody said. The prosecutor declined to say why Fox might have ignited the device during the party. The fresh revelations about the case came on Thursday as Fox pleaded not guilty to charges of murder, explosion of a destructive device causing death and child endangerment. When asked why Fox has been charged with murder if he did not intend to kill Ogara, 38, the prosecutor responded that the investigation is ongoing. Fox was arraigned in El Cajon, California, following his arrest Tuesday over the fatal nighttime explosion hours earlier at the couple's trailer home, which is less than 40 miles east of San Diego and across the border from Tecate, Mexico. The couple's 4-year-old daughter was inside the home, which is why Fox is charged with child endangerment, Mody said. Authorities have said Fox used fireworks powder in the weapon. He had the tube pointed away from the trailer home, which is located in a mountainous and rural area, but a portion of the pipe exploded as fragments pierced the wall of the trailer and struck Ogara in the upper chest, Mody said. She died "pretty much instantaneously," he said. The couple's three friends who were also in the trailer were not injured, but Fox was treated for wounds to his leg. Loyola Law School professor Stan Goldman said this early in the case, prosecutors do not have to indicate whether they are seeking a first-degree or second-degree murder charge. They are most likely aiming for second-degree murder, which is defined as not pre-meditated, under "a gross recklessness theory," he said. Fox, who remains in jail on bail of $3.5 million, made his court appearance from detention via video conference. His deputy public defender entered his not guilty pleas. Fox, who is due back in court on March 16, faces a maximum sentence of life in prison if convicted. Visibly upset family members of both Fox and Ogara attended the hearing on Thursday. Writing by Alex Dobuzinskis; Editing by Greg McCune