U.S. officials have told Sen. Bernie Sanders that Russia is attempting to help his presidential campaign as part of an effort to interfere with the Democratic contest, according to people familiar with the matter.

President Trump and lawmakers on Capitol Hill have also been informed about the Russian assistance to the Vermont senator, according to people familiar with the matter, who spoke on condition of anonymity to discuss sensitive intelligence.

It is not clear what form that Russian assistance has taken. U.S. prosecutors found a Russian effort in 2016 to use social media to boost Sanders campaign against Hillary Clinton, part of a broader effort to hurt Clinton, sow dissension in the American electorate and ultimately help elect Donald Trump.

“I don’t care, frankly, who Putin wants to be president,” Sanders said in a statement to The Washington Post. “My message to Putin is clear: stay out of American elections, and as president I will make sure that you do.