DENVER (CBS4) ― CBS4 has now learned at least four people are under arrest in connection with a possible plot to kill Barack Obama at his Thursday night acceptance speech in Denver. All are being held on either drug or weapons charges. CBS4 Investigator Brian Maass reported one of the suspects told authorities they were "going to shoot Obama from a high vantage point using a ... rifle … sighted at 750 yards." Law enforcement sources tell Maass that one of the suspects "was directly asked if they had come to Denver to kill Obama. He responded in the affirmative."...