The nation’s spy chief called on former intelligence contractor Edward Snowden to return a massive trove of classified documents on Wednesday during a congressional hearing on security threats that was dominated by heated exchanges over that security breach and the surveillance programs subsequently exposed.Speaking before a Senate panel, James R. Clapper Jr. outlined an array of dangers to American interests, including a rise in cyberthreats and the emergence of Syria as a magnet for Islamist militants linked to al-Qaeda.But after skimming through those developments, Clapper focused his opening remarks on Snowden, delivering a blistering stream of criticism in which he described the former contractor for the National Security Agency as a hypocrite who has severely undermined U.S. security.