U.S. President Barack Obama has vowed to retaliate against Russia for trying to interfere in the U.S. presidential election but hasn’t said exactly what he has in mind with just a month to go before he leaves office.

At his news conference Friday, Obama said U.S. intelligence agencies agree Russia was behind the hacking into the email accounts of the Democratic National Committee and Clinton campaign Chairman John Podesta. Both the FBI and CIA said the purpose of the hacking was to get Donald Trump elected president, a conclusion Trump rejects.

In an NPR interview, Obama said the U.S. would retaliate against Russia “at a time and place of our own choosing. Some of it may be explicit and publicized; some of it may not be." Putin spokesman Dmitri S. Peskov said the U.S. should show proof of Russia’s actions or shut up about it. However, much of the evidence cannot be released without exposing how the U.S. obtained it.

“We will provide evidence that we can safely provide that does not compromise sources and methods.  But I’ll be honest with you, when you’re talking about cybersecurity, a lot of it is classified.  And we’re not going to provide it because the way we catch folks is by knowing certain things about them that they may not want us to know, and if we’re going to monitor this stuff effectively going forward, we don’t want them to know that we know,” Obama said at the news conference.

White House spokesman Josh Earnest said all 17 U.S. intelligence agencies concluded the hacking was orchestrated by “Russia’s senior-most officials,” and Obama put it squarely in Putin’s lap.

“Not much happens in Russia without Vladimir Putin,” Obama said.

Obama has ordered a full investigation, and Senate Majority Leader Chuck Schumer, D-N.Y., joined a bipartisan group of senators calling for a select committee to investigate.

Obama said Trump should support such an investigation.

Here are six things the U.S. could do in retaliation as suggested by the Atlantic and Reuters:

  • Cyberattack on Russian networks or infrastructure
  • Release damaging information about Vladimir Putin
  • Target offshore accounts
  • Place malware inside Russian espionate networks
  • Interfere in Russian politics
  • Economic sanctions