The body of Spc. Megan Touma, 23, was found June 21 in a room at the Fairfield Inn near Cross Creek Mall. Investigators said they believe she was killed seven to eight days earlier.
Sgt. Edgar Patino, 27, of 374 Lairgate Lane in Hope Mills, was charged Tuesday with first-degree murder in her death. He is being held in the Cumberland County Jail without bond.
The source familiar with the investigation said additional arrests could come as early as next week.
Touma was pregnant, and Patino was the father of her unborn child, police said.
Police have declined to discuss a possible motive in the case or the cause of death, which they said would have to be released by North Carolina and Army medical examiners.
Patino had been the prime suspect in the investigation for several weeks, police said, based on evidence collected from the motel room and information provided by Army personnel at Fort Bragg and in Germany, where Touma and Patino were previously stationed.
Patino's wife was present when investigators searched the couple's Hope Mills home a few weeks ago, according to a neighbor.
Detective Joshua Carter, who is heading the investigation for the Fayetteville Police Department, declined to comment Wednesday about what Patino's wife might have known about Touma or her death.
Police said military investigators are still interviewing people at the Army post in Germany, and Carter and his team likely won't finish their investigation for several months.
Patino, who joined the Army in 2004, was assigned to the John F. Kennedy Special Warfare School at Fort Bragg, part of Army Special Operations, in December and was being trained in psychological operations.
Fort Bragg officials said Touma requested an assignment to the post in February and arrived on June 12. She wasn't seen on post after that, and officials said Thursday an Army investigation has been completed into why she wasn't declared absent without leave but that the findings haven't been released.
Carter said investigators believe Touma was killed late on June 13 or early on June 14 because no key card was used to access the room after that. Patino admitted to police that he had been in the motel room on the night of June 13, Carter said.
A "Do Not Disturb" sign had been hung outside the door, so no motel employee entered the room for several days, police said.
Touma's body was found in a bathtub after a motel employee responded to guests' complaints about an odor coming from her room.
A week later, someone sent letters to The Fayetteville Observer newspaper and to police claiming to be her killer. The letters included a symbol at the bottom that was similar to one used by "the Zodiac," a California serial killer in the 1960s who was never caught.
Investigators believe Patino sent the letters to mislead police. Investigators said they found a typewriter in Patino's house and learned he had purchased it on June 23, the day before the anonymous letters were postmarked.