ALEDO, Texas - A North Texas sheriff says "it's a big mystery" why a 17-year-old boy gunned down his mother and teenage sister in their home.

Parker County Sheriff Larry Fowler said Friday that Jake Evans remains jailed without bond on a capital murder charge.

During a 25-minute call to 911 early Thursday, Evans calmly says shooting his mother and 15-year-old sister "just kind of happened."

Evans tells the Parker County 911 operator that his mother and sister were rude but also says his sister had a really sweet side.

Evans tells the operator he'd "been kind of planning on killing for a while now" and he could have shot "pretty much just anybody."

The double slaying happened at the family's upscale home in Aledo, about 20 miles west of Fort Worth.