"We have a responsibility to conduct a thorough investigation to determine if she's a victim of foul play or if she's voluntarily missing," Jodi Miller, spokesperson for the San Bernardino County Sheriff's Department, tells PEOPLE. "She has not been located."
As the search for Erin continues, questions surround the nature of her relationship with her neighbor, former Marine Cpl. Christopher Lee.
Erin and her husband, Marine Cpl. Jon Corwin, had been living on base in Twentynine Palms, California, for less than a year when, in an effort to make new friends, the young couple started hanging out with their neighbors Christopher and Nichole Lee.
But the friendship ended as quickly as it started. Last fall, Nichole and Erin, 19, often rode horses at the White Rock Horse Rescue ranch in nearby Yucca Valley.
"After February, they never came together," owner Isabel Megli tells PEOPLE. "[Nichole] never said a word."
On June 28, Erin – who was nearly two months pregnant – told her husband she was heading to Joshua Tree National Park to check out scenic places for her and her mother, who was expected to arrive the following week, to visit. She never returned home and her husband called police the following day.
While more than 60 volunteers searched hundreds of square miles of scorched terrain around the park, detectives spoke with a childhood friend of Erin's who forwarded texts to police suggesting Erin and Christopher were having an affair and the baby could possibly be his, according to an affidavit.
At first, Christopher denied knowing much about Erin, but after being questioned again by police, he admitted to kissing Erin but nothing more.
"Lee stated his wife knew he and Erin were becoming too close," according to the affidavit.
More on the couple's relationship and the day Erin Corwin went missing is available in this week's issue of PEOPLE, on newsstands Friday