Authorities say 20-year-old Maria Frances Lauterbach was reported missing Dec. 19 by her mother, who was the last person to speak with her, on Dec. 13. Her cell phone was found Dec. 20 near the main gate at Camp Lejeune.
She is with the 2nd Marine Logistics Group and is about eight months pregnant.
The sheriff's department said Lauterbach's mother reported that her daughter was supposed to testify about an incident she witnessed on base.
"There are several findings and pieces of evidence that have been discovered that cause law enforcement to be concerned with the circumstances surrounding Maria's disappearance," the sheriff's department said in a news release.
The State Bureau of Investigation also is working on the case.
First Lt. Richard Ulsh, a public affairs officer with the 2nd Marine Logistics Gropu, verified that Lauterbach is on active duty. Marine Corps and U.S. Navy investigators are cooperating with the investigation, he said.
