NEW : Whistleblower says an armed contractor was allowed on an elevator with Obama . The White House has denied the allegations and said it is investigating them. Rep. Darrell Issa cites instances of `` misbehavior '' that undermine trust in Secret Service .
NEW : Whistleblower says an armed contractor was allowed on an elevator with Obama . Secret Service chief says agents used `` restraint in making a very difficult decision '' Gonzalez entered White House via unlocked door , Pierson tells congressional committee . Rep. Darrell Issa cites instances of `` misbehavior '' that undermine trust in Secret Service .