Mr. Flynns ties to Russian officials stretch back to his time at the Defense Intelligence Agency, which he led from 2012 to 2014. There, he began pressing for the United States to cultivate Russia as an ally in the fight against Islamist militants, and even spent a day in Moscow at the headquarters of the G.R.U., the Russian military intelligence service, in 2013.
He continued to insist that Russia could be an ally even after Moscows seizure of Crimea the following year, and Obama administration officials have said that contributed to their decision to push him out of the D.I.A.
But in private life, Mr. Flynn cultivated even closer ties to Russia. In 2015, he earned more than $65,000 from companies linked to Russia, including a cargo airline implicated in a bribery scheme involving Russian officials at the United Nations, and an American branch of a cybersecurity firm believed to have ties to Russias intelligence services.
The biggest payment, though, came from RT, the Kremlin-financed news network. It paid Mr. Flynn $45,000 to give a speech in Moscow, where he also attended the networks lavish anniversary dinner. There, he was photographed sitting next to Mr. Putin.
A senior lawmaker said on Monday that Mr. Flynn misled Pentagon investigators about how he was paid for the Moscow trip. He also failed to disclose the source of that income on a security form he was required to complete before joining the White House, according to congressional investigators.
"frequently, those who go along a treasonous path do not even realize they're along that path until it gets to be a bit too late."