A search for an "endangered" missing girl, whose parents were found shot dead in their home in the US state of Wisconsin, has expanded nationwide as police struggle to find clues to her whereabouts.Thirteen-year-old Jayme Closs has been missing since Monday, when an emergency call led authorities to their rural home in the Midwestern state. Her parents had been shot to death and the teen was gone. She is believed to have witnessed the murders, but is not considered a suspect based on evidence recovered at the scene, police said. An intensive search in her home state has failed to locate her despite involving some 100 people including volunteers and more than 800 phoned-in tips. "Nothing of evidentiary value has been recovered," the Barron County Sheriff's Department said in a Thursday statement. The FBI expanded the search nationwide on Thursday, describing Closs as "endangered." The agency began posting information about the teen on digital billboards around the country. "An entire state has been racked with Jayme's disappearance, and the death of her parents," Wisconsin state Attorney General Brad Schimel said in a statement.Closs disappeared after police received a 911 call from a cellphone inside her home at around 01:00 (0600 GMT) on Monday. Her parents James Closs, 56, and Denise Closs, 46, were found shot dead. Police don't know what happened or the motive for the killings, and a murder weapon has not been recovered, according to US media reports. CBS News reported that the family's front door had been shot open.