RAPID CITY, S.D. (AP) — A South Dakota teenager charged with killing a girl whose body was found in his basement shot her after an argument, a prosecutor alleged Wednesday.

The 17-year-old defendant is charged as an adult with second-degree murder, which carries a mandatory life sentence upon conviction. The teen made his initial court appearance Wednesday and will remain in juvenile detention with his bond set at $1 million.

The teen is charged in the death of the girl, whose body deputies found Monday in the basement of the teen's home near Sturgis, which is in western South Dakota just north of Rapid City and near the state's border with Wyoming. Authorities believe it is 16-year-old Shayna Ritthaler, of Moorcroft, Wyoming, who has been missing since last Thursday. Moorcroft is about 70 miles west of Sturgis.

Michele Bordewyk, the acting Meade County state's attorney, said in court that the defendant was with the victim when an argument turned violent and "he shot her," the Rapid City Journal reported.

Defense attorney Steven Titus, of Gillette, Wyoming, told The Associated Press that he will ask that the case be returned to juvenile court and that his client will plead not guilty at his next court appearance, which is scheduled for Oct. 17. The Associated Press isn't naming the defendant because his case might end up back in juvenile court.

Meade County Sheriff Ron Merwin said he couldn't yet share details on how the girl died. An autopsy is scheduled for Thursday. He told the newspaper that he doesn't know how the defendant and victim knew each other.

The body was found when deputies and agents from the South Dakota Division of Criminal Investigation executed a search warrant on the request of the Lawrence County Sheriff's Office, which was working with investigators in Wyoming, the sheriff's office said in a Facebook post.

Bordewyk said the boy lived at the home with his mother and that he had recently moved there from Brookings, which is in eastern South Dakota near the state's border with Minnesota.

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