US Defense Secretary Chuck Hagel acknowledged on Wednesday that authorities missed some \"red flags\" that might have prevented the deadly mass shooting at the Washington Navy Yard. \"Obviously, when you go back in hindsight and look at all this, there were some red flags, of course there were,\" Hagel told a news conference. \"And should we have picked them up? Why didn\'t we? How could we have? All those questions need to be answered.\"