No single measure can undo the change in social attitudes driving political threats, according to the head of the Australian Federal Police, as it grapples with how to keep parliamentarians safe.Commissioner Krissy Barrett began her late-night appearance at the Senate estimates hearing noting "a heaviness that hangs across Australia" after the attack on independent MP Helen Haines.The federal member for Indi lost both her legs and remains in a serious but stable condition in a Melbourne hospital after being struck repeatedly by a four-wheel drive outside her Wangaratta office.State authorities are leading the ongoing investigation into the motive behind the...