On Sunday, U.S. Marine Corporal Wayne Corwin started calling family members to see if they had heard from Erin, who had not been seen since leaving for Joshua Tree on Saturday.
"It was immediately concerning because she had already been gone for a day," Erin's sister-in-law, DeeAnna Heavilin told People Magazine.
"She didn't start driving until she was 18 years old and she is 19 now and she has a terrible sense of direction, so our first thought was that she was lost."
Police located Corwin's 2013 Toyota Corolla but said it gave no indication as to her direction of travel or eventual whereabouts, leading investigators to conclude her disappearance has "definite suspicious circumstances," so much so that homicide detectives have been deployed.
"The specialized division was asked to assume the investigation because they felt that there were definite suspicious circumstances," San Bernardino County sheriff's spokeswoman Cindy Bachman told local reporters. The division specializes in "investigations that involve the possibility of foul play," she explained.
"They really don't know where she is or what happened to her, and there is the potential for foul play."