President Trump's national security adviser Michael Flynn resigned Feb. 13 after revelations that he had discussed sanctions on Russia with the Russian ambassador to the U.S. prior to Trump taking office. Here's what you need to know. (Deirdra O'Regan/The Washington Post)

President Trump on Tuesday blamed “illegal leaks” for the downfall of national security adviser Michael Flynn, who resigned late Monday amid reports of potentially illegal interactions with the Russian ambassador to the United States before Trump was sworn in as president.

In a tweet, Trump expressed frustration with what he views as a culture of leaks in the nation’s capital. “The real story here is why are there so many illegal leaks coming out of Washington?” he wrote. “Will these leaks be happening as I deal on N. Korea etc?”

Flynn had originally denied — including to Vice President Pence — having privately discussed U.S. sanctions against Moscow with Russian Ambassador Sergey Kislyak during the transition period. Pence, based on Flynn’s assurances, then publicly defended him. But amid reporting that Flynn had discussed sanctions with the Russian envoy, the national security adviser backtracked on Thursday, saying he could not be sure the topic had not come up.

In his resignation letter Monday, Flynn said that he had apologized to both Trump and Pence, and that he “inadvertently briefed the Vice President Elect and others with incomplete information regarding my phone calls with the Russian ambassador.”

The president's tweet echoed the messaging of some conservatives close to the White House, who focused heavily on how the call between Flynn and the Russian official was intercepted and how that intelligence then leaked to the media.

Less than 24 hours after former national security adviser Michael Flynn resigned, some Republicans were calling for Americans to "move on," while at least one senator still had questions. (Sarah Parnass/The Washington Post)

“I think this really was the death by a thousand leaks,” Laura Ingraham, a conservative news commentator, said on Fox News. “The leaks that were coming out of this administration and the transition — before the administration — were at a level that I don't remember seeing for quite some time.”

Not long before Trump tweeted, Sen. Ron Johnson (R-Wis.), chairman of the Senate Homeland Security Committee, said on Fox that “somebody in the nebulous intelligence community” would have had access to the information about Flynn's calls.

“Who tapped the phones? Who is listening to it? Who leaked it? I think those are legitimate questions to ask,” Johnson said Tuesday morning.

The senator said he did not know whether those who leaked the information about Flynn broke the law, but he added: “Leaks of this nature are incredibly damaging to America, to our national security, and we need to look into it.”