A married Maryland man has been federally charged with kidnapping and killing his girlfriend after luring her to the Nevada desert, where he says he tied her to a post and suffocated her last year, according to new court documents.

The FBI charged John Matthew Chapman, 39, with kidnapping resulting in death in a complaint filed Monday in U.S. District court, news outlets in Nevada and Pennsylvania reported.

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Chapman is accused of kidnapping and murdering 33-year-old Jamie Rae Feden, of Bethel Park, Penn., last September. Her body was identified in November, a month after her remains were found in the desert.

Chapman was arrested in November in Pennsylvania, and told police he took Feden on a cross-country road trip to Las Vegas to look at homes, FOX5 Las Vegas reported, citing court documents.

Chapman came up with the plan to kill Feden before they left Pennsylvania, and even had a “kill kit” ready, the document said. Authorities have alleged his motive was money.

He told police he lured Feden out to the desert under the guise of a bondage photoshoot, according to the documents. He told police he used zip ties to bind Feden’s hands and feet and tied her to a signpost. He then covered her mouth and nose with duct tape and watched her suffocate.

He removed the zip ties, tape and Feden’s clothing before leaving her body near the signpost and driving back to Pennsylvania, the documents stated.

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Feden was just over 4 feet tall and weighed 75 pounds, police said. She suffered from VATER syndrome, a rare congenital condition. The letters stand for vertebrae, anus, trachea, esophagus and renal (or kidneys), which are the areas of the body that are affected.

While Feden was missing, the FBI said Chapman lived in her house and sent her family messages on Facebook to cover up her death, the Pittsburgh Post-Gazette reported.

After Chapman was taken into custody, he called his wife, Maureen, and said, “I killed her because I had to,” Maureen told WPXI-TV at the time. She said her husband was living a “double life” and had told his family he was going to Las Vegas on a work trip.

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Chapman has remained in custody since his arrest. He will appear before a judge next month.