FAYETTEVILLE, N.C. – The Army's criminal investigations unit is joining the effort to catch the killer of a pregnant Fort Bragg soldier who was found dead in the bathtub of a motel room, authorities said Monday.

Fayetteville police said civilian and military detectives are now treating last month's death of Army Spc. Megan Touma as a homicide, although they have not determined the cause of her death. Authorities have a person of interest in the case, a soldier the military has said is assigned to the Fort Bragg-based U.S. Army Special Operations Command.

“It made sense. We've been providing information and working with Fayetteville from the start,” said Chris Grey, a spokesman for the Army's Criminal Investigation Command.

They're also investigating a letter sent to the local newspaper, purportedly from the killer, that included a symbol similar to one used by the famed Zodiac killer of the 1960s. Police said Monday that a similar symbol was found on a mirror in Touma's motel room.

Touma, 23, of Cold Spring, Ky., was found June 21 at a Fayetteville motel after a maintenance supervisor smelled a foul odor coming from a room with a “do not disturb” sign. The dental specialist was seven months pregnant and had recently arrived from a base in Germany. She was temporarily assigned to the base's 19th Replacement Company, and would have eventually been assigned to work at a base dental clinic.

Grey said the Army was already monitoring the city's investigation into Touma's death when it formally joined the effort to solve the case. Fayetteville police spokesman Lt. David Sportsman said the police are still leading the investigation, but Army investigators have “worldwide” access to soldiers and are better able to gather information in Germany.

“As this investigation has matured and because of the obvious link to the U.S. Army, it was expected that Army CID (criminal investigators) would eventually work this case jointly with our agency,” Sportsman said in a statement.

Police said Monday that neither the state medical examiners nor the Armed Forces Institute of Pathology have determined Touma's cause of death. The Army took possession of her body last week, with plans to perform a second autopsy and additional tests.

There was no mention Monday in Sportsman's statement of a letter sent to The Fayetteville Observer. Sportsman had previously said authorities believe the author's confession of being a serial killer was an attempt to mislead investigators and the media.

The letter included a symbol similar to one used by the Zodiac killer, who was blamed for at least five slayings in California in the late 1960s but never caught.

“I will start using my role-model's signature,” the author says in the letter, which was posted on the newspaper's Web site.