An autopsy on Tuesday left investigators no closer to finding out what caused the death of 19-year-old Alyssa Byrne, whose body was found Friday after a three-day search.El Dorado County Sheriff's Lt. Pete Van Arnum said Wednesday that the autopsy did not reveal a cause of death."Cause of death is pretty much pending toxicology, which could take 4-6 weeks," he said. "There was no obvious sign of death that could be determined."Van Arnum said Byrne's jacket was found with her, but she wasn't wearing it when she was found on Friday.Byrne, a Petaluma, Calif., resident, last used her cell phone …