A gunman who died after a standoff with police at a Nebraska pharmacy admitted killing a Colorado man last December, authorities said Thursday. The Nebraska State Patrol said 27-year-old Andres Gonzalez told negotiators during Tuesday's standoff that he killed Joshua Bullock, whose burned pickup truck was found by a rancher Dec. 14 near Chadron. The western Nebraska town is about 50 miles north of Alliance, where the pharmacy shootout took place. Investigators believe Gonzalez killed the 38-year-old Denver man around Dec. 1 and left his body in a remote area of western Nebraska, though his remains haven't been found, State Patrol spokeswoman Deb Collins said in a statement. Authorities arrested Gonzalez's girlfriend, 19-year-old Rose Siefke, of Hemingford, on a charge of being an accessory to a felony, according to the State Patrol's statement. She was being held at the Box Butte County Jail, where officials couldn't provide any information including whether Siefke had an attorney. County Attorney K.J. Hutchinson said investigators believe Siefke helped conceal Bullock's death, but she had not been formally charged as of Thursday. Authorities said Gonzalez tried to rob Thiele Pharmacy & Gifts early Tuesday, triggering a 14-hour standoff. Gonzalez died after a shootout with police. Earlier in the day, officers found his father dead at home. Three police officers and the pharmacist were shot and wounded, and they remained hospitalized Thursday. Patrol officials said investigators were trying Thursday to piece together the incidents surrounding Bullock's death and the shootout. "Josh Bullock's disappearance was an active case, but we had little information to go on," Lt. Monica Bartling said. "Our investigation will now focus on finding Josh's remains and bringing some closure to his family and friends." Bullock had ties to Chadron and Alliance, and he was last seen in Alliance after visiting family on Dec. 1. Autopsies were conducted Thursday on Andres Gonzalez and his father, Larry Gonzalez. Hutchinson said it was too early to say how either man died. The two officers and pharmacist wounded in the standoff were in stable condition Thursday, Box Butte General Hospital spokeswoman Bonnie Wallace said. In Alliance, a city of about 8,600 people, restrictions had been lifted on access to downtown. Cheryl Moser, who manages the Dollar General store down the block from the pharmacy, said her customers were returning Thursday, and some people seemed to be looking for bullet holes near the shooting scene. "Everybody's curious if they weren't down here," she said.