Authorities are investigating a possible assassination plot against presumptive Democratic presidential nominee Barack Obama. At least four people have been arrested in connection with a possible plot to kill Obama at his Thursday night acceptance speech at Invesco Field at Mile High, according to CBS4 News. The suspects are being held on either drug or weapons charges. Aurora police arrested a longtime drug user Sunday afternoon during a routine traffic stop where the man was seen "weaving," sources said. Three possible other accomplices also were arrested, according to police.