President Barack Obama put Russia’s Vladimir Putin on notice Friday that the U.S. could use offensive cyber muscle to retaliate for interference in the U.S. presidential election, his strongest suggestion to date that Putin had been well aware of campaign email hacking. Caught in the middle of a post-election controversy over Russian hacking, Obama strongly defended his administration’s response, including his refusal before the voting to ascribe motive to the meddling or to discuss now what effect it might have had.