One of the most disturbing stories to come out of the brutal 2016 U.S. presidential election cycle was that Russian operatives, publicly encouraged by then-candidate Donald Trump, had allegedly hacked the Democratic National Committee in an attempt to tip the scales towards Trump, a much more pro-Russia candidate than former Secretary of State Hillary Clinton.

In the wake of the election, significant evidence has emerged of further inappropriate actions by Trump’s campaign team and Russia — actions that could easily be characterized as treasonous. On Tuesday, for example, The New York Times published a story revealing private contact between Russian intelligence officials and senior members of Trump’s team, including former Campaign Manager Paul Manafort, throughout the course of the election.

Another troubling piece of evidence is the recent exposure and subsequent resignation of National Security Adviser Michael Flynn, who announced Monday night that he would step down from his post after only 24 days. The resignation came quickly on the heels of news that Flynn had potentially misled members of the Trump administration, including Vice President Mike Pence, about the details of his conversations with Sergey Kislyak, the Russian ambassador to the U.S., that took place during the final weeks of the Obama administration.

The conversations between Flynn and Kislyak concerned how relations between the U.S. and Russia would change under the Trump administration, specifically regarding the lifting of sanctions implemented by the Obama administration after learning that Russia had been behind the hacking of the DNC. Flynn also discussed possible areas of cooperation between the two countries.

At face value, potential collusion between Flynn and Russia is troubling. Russia interfered in November’s election after being egged on publically by Trump — for a member of the Trump team to privately discuss lifting the sanctions that were imposed in retaliation for those actions threatens the integrity of our democratic process.

Therefore, in the wake of Flynn’s resignation, it is important to examine these questions: How much did Trump know about Flynn’s communications with Russia as they were happening? Is it truly possible that Trump was ignorant of conversations between Russia and his own national security adviser?

The answers to those questions are inevitably damning. Either Trump actively encouraged one of his top advisers to place an inappropriate phone call with the Russian ambassador or he has lost control of his administration.

Following Flynn’s resignation, California Representative Adam Schiff, the ranking member on the House Intelligence Committee, released the following statement: “The Trump administration has yet to be forthcoming about who was aware of Flynn’s conversations with the Ambassador and whether he was acting on the instructions of the President or any other officials.”

In an interview Tuesday, Schiff doubled down on his statement, saying that it was “unlikely” that Flynn was “acting as a free agent.”

It is worth noting that Flynn’s resignation is not the first of its kind among Trump’s inner circle — Manafort also resigned last August in the midst of a controversy over his own connections to Russia. The common thread? Both men had ties to the Kremlin and both had Trump’s ear.

The possibility that Trump was fully aware of Flynn’s phone calls is not a baseless conspiracy theory, especially as reports are now surfacing that the Department of Justice notified the Trump administration a month ago that Flynn had participated in a potentially troubling conversation with the Russian ambassador.

In spite of the Justice Department’s warning, Flynn remained in the President’s highest confidence until the end, according to Trump advisor Kellyanne Conway hours before Flynn’s resignation.

Given the copious evidence of suspicious activity between Russian officials and several senior members of Trump’s team, Flynn’s resignation must prompt a renewed commitment to investigating Trump’s ties to Russia. It is important that the American people know exactly how much Trump knew about Flynn’s communications with Russia and exactly when he knew it.

At this point, the sum of the known evidence gives great cause for concern. If Trump is truly concerned with being tough on Russia, as White House Press Secretary Sean Spicer claims, then it is difficult to understand why his administration ignored warnings from the Justice Department that Flynn was having troubling conversations with Russia that Trump claimed to be ignorant of.

The pressure on Trump to be upfront and honest about his administration’s dealings with Russia must be a bipartisan effort. Democratic leaders have already affirmed their commitment to get to the bottom of Russian interference in the 2016 election. Republican Senators Lindsey Graham and John McCain have as well. Now is the time for Republicans to get tough not only on Russia but on Trump himself.