FARGO, N.D. -- Fargo police have opened a criminal investigation into the disappearance of a 22-year-old pregnant woman.

Savanna Greywind was last seen Saturday afternoon, when she went to help a neighbor who needed a model while sewing a dress, reports the Duluth News-Tribune.

Fargo Police say she is eight months pregnant.

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Greywind's mother told local media she doesn't believe her daughter left on her own. Norberta LaFontaine-Greywind told the paper her daughter left behind her car and her wallet. She said Savanna had ordered a pizza before going upstairs to help her neighbor, but never returned to eat it.

"I immediately knew something was wrong because her car is here," her mother told the paper. "She's eight months pregnant. Her feet were swollen, so she wouldn't have taken up walking like that. There was pizza here that she hadn't eaten. She would not just leave that lady's apartment and go somewhere."

The Greywind family is offering a $7,000 reward for information that leads to solving the case. LaFontaine-Greywind told the paper her daughter is "very, very" excited about becoming a mom.

"She's in danger," LaFontaine-Greywind said. "Something's wrong."

Fargo police said in a statement released on Facebook Thursday that investigators have spoken to Grewywind's family, boyfriend, neighbors, employer, and "anyone the family has mentioned as a friend." The department said they attempted to ping her cellphone and have conducted two K9 searches, but haven't turned up anything.

The Fargo Fire Department also conducted a river search and an aerial search with the assistance of U.S. Border Patrol, police said, also with no results.

Family and friends gathered Wednesday in downtown Fargo for a vigil and prayer service.

Anyone with information on the case is asked to call call the Fargo Police Department at 701-235-4493, text a tip to 701-730-8888 or call the Fargo Police Tip Line at 701-241-5777.