Corwin was about three weeks pregnant when she disappeared June 28 from her Twentynine Palms, California, home. Her husband, Marine Cpl. Jonathan Corwin, reported her missing the next day.
The investigation grew to focus on Lee, 24, who was the Corwins' neighbor, according to court papers first obtained last month by the Desert Sun newspaper.
Erin Corwin's friend told investigators that Lee and Erin Corwin were having an affair and that the unborn child might be Lee's, the papers show. The friend showed authorities text messages that said Erin Corwin was planning to go on a special hunting trip with Lee to celebrate on the day she disappeared.
Authorities have been searching for Corwin for weeks and have not announced whether her body has been found.
"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."
Lee, a former Marine, told investigators he was not with Corwin that day and said that although the two had kissed, they had never had intercourse.
It wasn't immediately clear if Lee, who has yet to make a first court appearance or arrive in California, had retained an attorney.
Lee was arrested last month on suspicion of possessing a destructive device after a search warrant was served at his Yucca Valley home. He was taken into custody after the July 4 search and released on bail two days later.
He was then honorably discharged after seven years in the Marine Corps and subsequently moved his family to Alaska.