Authorities believe a Wisconsin teen arrested Sunday morning in connection with the murder of his grandparents had a bigger plan: to “harm” his high school.

Neenah High School junior Alexander Kraus was arrested for the double murder after Grand Chute police officers performed a welfare check at a home located about two hours outside of Milwaukee. Inside, officers discovered the 17-year-old and two bodies, who police confirmed Monday were Dennis and Letha Kraus, his grandparents.

“The teenager was still at the scene when police arrived and was brought back to the station for questioning, he was cooperative,” Grand Chute police officer Travis Waas said Monday. “This was a terrible situation but he is being cooperative in this investigation.”

Waas also confirmed police recovered a “long gun” at the scene on Sunday, which authorities now believe was used in the crime.

Kraus was taken into custody just after noon on Sunday on two counts of first-degree intentional homicide and was booked into Outagamie County Jail. According to the Associated Press, the teen allegedly admitted to shooting his grandparents. He is expected to have an initial court appearance on Tuesday afternoon, Grand Chute police said.

Authorities also confirmed Monday that an early investigation yielded “substantial” information, which includes written documentation, that Kraus planned to “cause harm” at his high school, but declined to comment further.

Neenah High School remained open on Monday, citing police insistence that this was an “isolated incident.”

“Grand Chute police informed the Neenah Joint School District that the student arrested on two counts of first degree intentional homicide on Sunday also had a plan to cause harm at Neenah High School,” the district said in a Monday statement obtained by The Daily Beast. “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.”