WARWICK, N.D. – The disappearance and discovery of 36-year-old Amanda Stach Engst's body is now being investigated as a homicide, according to the FBI.

Engst's body was found in early February in the Sheyenne River south of Warwick on the Spirit Lake Indian Reservation. Eddy County Sheriff Paul Lies said at the time the cause of death did not appear to be natural, though authorities would not identify her death as a homicide.

FBI spokesman Kyle Loven said Thursday the case is being treated as a homicide.

"The FBI, along with our various partners, the Bureau of Indian Affairs, the North Dakota Bureau of Criminal Investigation and the Eddy County Sheriff's Office, are conducting a joint investigation into the matter," Loven said. "We are proceeding in the manner that this very well could be a homicide."

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Loven said he could not discuss any details of the case, including if there are suspects, as it is an ongoing investigation.

Engst grew up in Breckenridge, Minn., and recently had lived in Fargo and Halstad, Minn., as well as Warwick.

Prior to the discovery of Engst's body, a possible link between her disappearance and a string of robberies in North Dakota and Minnesota was discovered. Engst's vehicle was found in the possession of Billy Joe and Crystal Marie Herman, both 36, when the two were arrested separately last year on charges of robbery and burglary.

The Hermans were sentenced in January in connection with an Oct. 30 bar robbery in Buxton, N.D. Billy Joe Herman is serving a one-year sentence at the state penitentiary in Bismarck, while Crystal Herman is incarcerated at the Northwest Regional Corrections Center in Crookston.

They face other charges connected to robberies or burglaries in Perley, Minn., and Grandin, N.D.