A Washington state man faces first-degree murder charges after allegedly killing his girlfriend in the heat of an argument and living with her body for at least two weeks, police said.

Chris Foiles, 42, of Spokane, was arrested last Friday in the presumed death of Megan Ashley Stedman and booked into Idaho's Bonneville County Jail without bail, the Idaho Falls Police Department wrote in a press release. 

Before his arrest on Friday, Foiles immediately waived his Miranda rights, shouting, "I am Chris Foiles, I killed my girlfriend, she is in the RV," USA Today reported. Police in Bozeman, Montana, said they had been searching for Stedman's boyfriend and her motor home since her disappearance on Dec. 15. 

Inside the vehicle, Idaho Falls Police found a woman's body with a tattoo resembling Stedman's and credit cards in her name, KRTV reported. An autopsy is underway to officially identify her, the Bozeman Police Department told Fox News Digital.

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Foiles told police that he and Stedman parked the motor home in a Walmart parking lot in Idaho Falls around Dec. 22. A "few weeks" before his Friday arrest, Foiles told police, he stabbed Stedman in the neck. Then, he "intentionally stabbed Megan in the chest with the intent to kill her," arresting documents reviewed by USA Today show. 

Foiles allegedly confessed to stabbing the woman several more times until she stopped moving. 

But family members of the 34-year-old nail technician told Fox News Digital that they knew something was horribly wrong when the mother-of-two never called on Christmas or her daughter's birthday. It was unlike her to go without contacting her children, they said, and they suspect she was killed earlier in the month. 

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Stedman's siblings said they searched tirelessly for the woman for weeks after her best friend contacted them, saying that she hadn't heard from Stedman. On New Year's Eve, Stedman's sister Marie Mitma said the family scoured the bars and holiday parties in the areas of Livingston and Bozeman, Montana – towns where Stedman had recently lived in her motor home. 

Meanwhile, police in Bozeman distributed photos of Stedman's missing RV with its red stripe and broken left tail light. Mitma said she spoke with the man who notified police of the RV shortly after he made the call. 

Foiles had repainted the vehicle about four weeks ago, Idaho Falls Police said – the good Samaritan allegedly told Mitma that he nearly overlooked it. 

"The blue stripe that he put on the RV threw him off, but I feel like my sister was with him," Mitma said on Wednesday. "He felt like he wasn't going to call it in – but for some reason, he had a strange feeling… I'm thankful that he did."

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Mitma told Fox News Digital that Stedman was "trying to get out" of her relationship with Foiles, and that there were ample red flags before she disappeared.

Foiles was a three-time convicted domestic abuser, according to arrest records obtained by KBZK. Despite pending charges, interim Livingston, Montana, Police Chief Wayne Hard told the outlet, Foiles was free on bail. 

"Her friends [were] really wary of him," Mitma said on Wednesday. "[Stedman] was afraid for her life, and she told people she worked with that she was afraid for him to get out, that she was afraid he would kill her."

Hard told KBZK that he "would like to have seen [Foiles] still be in jail, obviously," and that "there's a lot that goes into all of that that I don't understand."

Stedman's beloved dog, Cali, is still unaccounted for – Mitma told Fox News Digital that her "biggest fear is that [Foiles] ended up killing the dog with her." 

"It's just disgusting. He could've just walked out," Mitma said. "She didn't deserve this and she didn't deserve to be killed. I wish law enforcement would take domestic violence seriously."

A mother to a daughter and son, Stedman became a nail technician at Glen Dow Academy in Spokane, Washington, then went back to become an instructor. She was the youngest of four siblings, and the family said she was "brave, independent and amazing."

"She was a very social butterfly," Mitma said. "Megan loved to cook, she always offered people to eat. She was an outgoing person – she loved to do things. She helped everybody out that she could, she was always happy, always smiley. Her number one priority was her kids."

Stedman's siblings have launched a GoFundMe to cover their sister's funeral expenses, their travel expenses as they attend upcoming court hearings and her children's welfare.