Alexander M. Kraus, 17, was arrested Sunday at his grandparent's home in Grand Chute after police responded to a welfare check and found the bodies of 74-year-old Dennis Kraus and 73-year-old Letha Kraus.
The Grand Chute Police Department said in a news release a 911 call had been placed from the home, but did not release details.
Kraus, who lives in nearby Neenah, admitted to investigators he shot the couple, Officer Travis Waas told the Associated Press.
After killing the couple, officials said the 17-year-old had additional plans for violence at Neenah High School, where he was a junior.
In a statement released to FOX11, the school district said that police informed school officials Kraus "had a plan to cause harm at Neenah High School."
"Police have indicated that there is no danger to students and staff at the high school and the school day on Monday will proceed as normal," the statement read. "Additional counselors are available to students and there is an extra police presence as an additional precaution."
Grand Chute, a city of 22,000, lies about 110 miles northwest of Milwaukee. Neenah, home to 26,000, is about 15 miles south of Grand Chute.
Wisconsin is one of six states that treat 17-year-olds as adults in the criminal justice system.
Kraus was being held in the Outagamie County Jail and has not yet been charged. Police said in a news release that he could be charged with two counts of first-degree intentional homicide, with each charge carrying the possibility of life in prison.
An initial court appearance was set for Tuesday afternoon, according to FOX11.
The Associated Press contributed to this report.