It was on federal land in Colorado. Pike's national forest.
You are absolutely right. Federal property would be a circumstance for a federal crime. I'm not sure exactly where the murders occurred on the trail, if it was private or state owned, or federal. I don't believe sexual assaults would be a federal offense unless they included kidnapping across state lines. From what I have read, the Monon Trail is owned by the state of Indiana, so it's still not a federal entity.