The Fort Bragg soldier was questioned after the incident which shut down the busy highway leading to the military post for hours.
Frances Sanchez, 21, was rushed to the hospital, but died from her injuries Tuesday.
The ex-husband has not been identified by police.
Witnesses told ABC11 Monday that they heard a gunshot, but didn't witness the actual shooting.
"When the officer arrived, there was a suspect sitting in the driver's seat of the car," said Gavin MacRoberts, Fayetteville Police Department spokesperson in an interview earlier this week. "The victim was lying outside the passenger side. There were several people around the victim working on her at the time the officer came up to talk with the suspect and took him into custody at that time."
Police say based on autopsy results, they believe Sanchez shot herself.