Sen. Bernie Sanders (I-VT) on Thursday pressed for more information on the ongoing FBI investigation into possible collusion between the Donald Trump campaign and Russian officials, asking why the president “has nothing but nice things to say about” Russian President Vladimir Putin.

“What do the Russians have on Mr. Trump?” Sanders asked.

How does it happen that we have a president who has nothing but nice things to say about Mr. Putin? What do the Russians have on Mr. Trump? — Bernie Sanders (@BernieSanders) March 23, 2017

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Sanders has been an active critic of the Trump administration, though he’s promised to work with the president on areas of common ground, including infrastructure. Recent developments in the FBI’s investigation into Russian interference in the 2016 election seems to have soured any semblance of good will between the men.