Corwin, a quiet Tennessee native who volunteers at a horse shelter, was last seen leaving her Twentynine Palms, California, home Saturday morning and was believed to be headed to nearby Joshua Tree National Park for a day trip in advance of her mother's upcoming visit, reports KESQ in Palm Springs.
Her husband, Marine corporal Jonathan Wayne Corwin, reported her missing on Sunday when she did not return to their home on a Marine base in the desert city.
On Monday, authorities discovered Corwin's 2013 Toyota Corolla in Twentynine Palms, but Corwin, who is three months pregnant, was not inside. Police did not reveal the exact location of the car for fear of compromising the investigation and “there was no indication of her direction of travel or current whereabouts," according to a news release issued Wednesday by the San Bernadino Sheriff's Department.
The sheriff's department has not ruled out foul play and homicide investigators are also investigating her disappearance.
The SearchApproximately 30 search and rescue trained volunteers are searching for Corwin in Joshua Tree National Park, a vast desert park that includes "very rocky, really dangerous areas," said Det. Marcel Silva.
Corwin's family members have set up a Facebook page to spread the word, and her mother, Lore Heavilin, is urging anyone with information to contact authorities, while she remains hopeful for good news.
"I mean my gut was just tore up, just tore up," Lore Heavilin told KESQ of the moment she discovered her daughter was missing. "And you feel like you're walking in a nightmare."
Erin's sister-in-law, DeeAnna Heavilin, tells PEOPLE Erin's husband called the family Sunday morning to say she was missing, and "it was immediately concerning because she had already been gone for a day. 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."
'Not an At-Risk Person'She describes Erin as a gentle, responsible person.
"Erin is the absolute bottom of my list for something like this to happen," she says. "She is not an at-risk person, she doesn't have a wild side, no drugs, no history of anything like that. She is a military wife who stays home and volunteers at a horse shelter. This is the last person who you would ever think that something like this could happen to."
DeeAnna tells PEOPLE her family was not aware Erin was pregnant, but believe she was waiting for her mother's upcoming visit to share the news.
"We believe that she was waiting to tell her mom – who is her best friend in the world – for when her mom was coming to visit her for her birthday, which is July 15."
Mom Lore said the pair had plans to visit Sea World and the San Diego Zoo during the trip. Yet all that's on hold until the family gets more information about Erin's mysterious disappearance.
"We are really, really confident in the team they have put together," says DeeAnna. "We want to know something. We want to get the call that they found Erin safe and sound. That's just what we are hoping for."
With reporting by ALICIA C. DENNIS