Jake Evans told the Parker County 911 operator early Thursday morning, "It's weird, I wasn't even really angry with them."
Authorities say he shot his mother, 48-year-old Jami Evans, and his 15-year-old sister, Mallory, at the family's upscale home near Aledo, about 20 miles west of Fort Worth.
He told the operator it "just kind of happened," and said he'd "been kind of planning on killing for a while now."
When the operator asked if he'd planned on killing them or "just anybody," Jake Evans replied, "Pretty much just anybody."
The teen was charged with capital murder and is jailed without bond.