An investigation is needed into a possible leak of classified intelligence from a briefing for lawmakers on Russian interference in the 2020 U.S. presidential campaign, Donald Trump said Sunday.

Trump paused as he left the White House for a trip to India and told reporters he had not been briefed on intelligence claims Russia was aiming to boost the campaign of Democratic presidential candidate Sen. Bernie Sanders (I-VT).

He then said that questions must be answered as to who did the leaking and why.

“They leaked it – Adam Schiff and his group, they leaked it to the papers as usual. They ought to investigate Adam Schiff for leaking that information. He should not be leaking information out of intelligence. They ought to investigate Adam Schiff,” Trump said.

As Breitbart News reported, U.S. officials have warned Sanders that Russia is attempting to enhance his prospects of winning the White House in 2020, sources familiar with the matter told the Washington Post.

President Donald Trump and Congressional lawmakers have also received briefings regarding the Kremlin’s efforts to help Sanders, reports the Post. In a statement to the newspaper, Sanders called Russian President Vladimir Putin a “thug” and said he stands “firmly against” any efforts to interfere in U.S. elections.

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

“In 2016, Russia used internet propaganda to sow division in our country, and my understanding is that they are doing it again in 2020. Some of the ugly stuff on the Internet attributed to our campaign may well not be coming from real supporters,” he added.

A spokesperson for the Sanders campaign would not comment on the nature of Russia’s alleged efforts to help the Vermont senator.