The search finally ended Saturday when a body was spotted with a video camera 140 feet down a mine shaft on federal land near Twentynine Palms, where she had been living.
In a little more than 24 hours, authorities had recovered and identified the remains of Erin Corwin and arrested her former neighbor and alleged lover, 24-year-old former Marine Christopher Brandon Lee, in Anchorage, Alaska, on suspicion of homicide.
"The manner of death has been determined to be homicide," San Bernardino County Sheriff John McMahon said Monday, adding that deputies searched many of the 100 mine shafts in a 300 square mile area before zeroing in on the one that contained Corwin.
Corwin, whose husband was stationed in Southern California, was in the early stages of pregnancy when she disappeared on June 28, according to court papers. Her husband, Marine Cpl. Jonathan Corwin, reported her missing the next day.
The investigation grew to focus on Lee, who was the Corwins' neighbor, according to court papers.
Erin Corwin's friend told investigators that Corwin and Lee were having an affair and that the unborn child might be Lee's, the papers show. Corwin told the friend, who lives in Tennessee, that Lee was worried his wife might divorce him and prevent him from spending time with his child if she knew Corwin was pregnant, the affidavit says.
Corwin told her husband when she left that day that she was going to scout out hiking trails in Joshua Tree National Park that she and her mother could explore when her mother visited a few days later. But the friend showed authorities text messages that said Corwin was planning to go on a special hunting trip with Lee the day she disappeared.
"It is highly likely that Erin could have been harmed by an unknown firearm," investigators wrote in the documents. "Sometime after Erin left with Lee, her phone was deactivated (turned off). Detectives believe if Erin was injured and left at an undisclosed location, she would not (be) able to call for help."
Two days after Corwin disappeared, investigators found her car abandoned off a road near her home. A single set of footprints led from the driver's side door and disappeared next to a set of vehicle tracks that matched the tires on Lee's Jeep. They found 10 spent casings from a .40-caliber gun in the Jeep, the court papers say.
Other evidence was found in the shaft one of more than 100 in the area but sheriff's officials declined to give details.
Authorities also declined to release the exact manner of death and said they could not confirm Corwin's pregnancy until an autopsy was completed.
Corwin would have been 20 if alive, but authorities say she likely died before her birthday.
Lee told investigators he was not with Corwin that day and had been hunting in Joshua National Park. He told police that although the two had kissed, they had never had intercourse.
The investigation found that Lee and Corwin did have an "intimate relationship" dating back to at least February, San Bernardino County Sheriff's Sgt. Trevis Newport said.
It wasn't immediately clear if Lee, who has yet to make a court appearance or arrive in California, had retained an attorney.
"Let's just wait until the facts come out before jumping to conclusions," Bob Lee, Christopher's father, told an Associated Press reporter during a brief interview outside the family's home in a working class neighborhood in Anchorage. He said the arrest came as his son and daughter-in-law, Nichole Lee, were returning home.
San Bernardino County District Attorney Michael Ramos said he expected to charge Lee with murder, but charges have not yet been filed.
Lee was arrested in July on suspicion of possessing a destructive device after a search warrant on a Yucca Valley home where he was temporarily living. He was taken into custody after the July 4 search and released on bail two days later.
He was honorably discharged after seven years in the Marine Corps and subsequently moved his family to Alaska.
Corwin and her husband are from Oak Ridge, Tennessee.
Law enforcement agencies including the FBI helped search for Corwin, whose body wasn't recovered until Sunday due to poor air quality conditions in the shaft. One firefighter was injured in the recovery effort, San Bernardino County Fire Chief Mark Hartwig said.
Corwin was found in a remote area some 130 miles east of Los Angeles, Newport said.
"It is very rough terrain out there. One would have a difficult time, even in a four-wheel drive vehicle," he said.
Flaccus reported from Tustin, California. Associated Press writers Brian Melley in Los Angeles and Rachel D'Oro in Anchorage, Alaska, contributed to this report.