Donald Trump has long pushed the theory, debunked by U.S. intelligence, that Ukraine interfered in the 2016 election. The Washington Post reports how Trump may have come to that view.

After meeting privately in July 2017 with Russian President Vladi­mir Putin at the Group of 20 summit in Hamburg, Trump grew more insistent that Ukraine worked to defeat him, according to multiple former officials familiar with his assertions.”

Many of Trump’s advisers, according to the Post, believe it was Putin himself who planted the Ukraine idea with Trump.

One former senior White House official said Trump even stated so explicitly at one point, saying he knew Ukraine was the real culprit because “Putin told me.”

Two other former officials said the senior White House official described Trump’s comment to them.

The Post interviewed 15 former administration and government officials for this story. All spoke on the condition of anonymity.