National Security Adviser and former Army Lt. Gen. Michael Flynn submitted his resignation to President Donald Trump Monday, after reports indicated Flynn discussed sanctions with the Russian ambassador during a December phone conversation and then lied about it to administration officials. In his resignation letter, Flynn admitted he "provided incomplete information" about the conversation.

Flynn has now become the third adviser to Trump, and the first since he assumed the presidency, to resign after facing scrutiny over ties to Vladimir Putin's Russia. In August, Trump campaign chairman Paul Manafort resigned just days after it was revealed that his name appeared on a secret ledger maintained by the administration of ousted pro-Russian president of Ukraine, Viktor Yanukovych. The ledger indicated Manafort received of millions of dollars in under-the-table secret payments from the former president, who is currently in exile in Russia. Then in September, Trump campaign foreign policy adviser Carter Page stepped down just days after Yahoo reported that intelligence officials were investigating whether Page had been communicating privately with senior Russian officials. 

Trump himself has been under fire for his repeated refusal to disavow Russian President Vladimir Putin. After the U.S. intelligence community said Russian hackers were behind cyber attacks against Democrats during the 2016 elections, Trump refused to publicly accept the assessment, going so far as to criticize U.S. intelligence services. 

"These are the same people that said Saddam Hussein had weapons of mass destruction," the Trump transition team said in a statement. Trump eventually admitted the Russians "probably" hacked the election during a January press conference in which he attacked Buzzfeed and CNN for reporting on an unverified intelligence dossier that claimed Russia had material with which it could use to blackmail Trump. 

Now the president faces questions about when he knew that Flynn had lied. On Friday, Trump said he didn't know about reports that Flynn discussed sanctions with Russia. But the Washington Post reported Monday that Sally Yates, the acting attorney general at the time, told the White House counsel that Flynn could be vulnerable to Russian blackmail after lying to his bosses about the contents of his phone conversation with the ambassador. Trump fired Yates on Jan. 31 for refusing to enforce his travel ban executive order. 

On Tuesday, Sen. John McCain (R-AZ) said in a statement that "General Flynn’s resignation also raises further questions about the Trump administration’s intentions toward Vladimir Putin’s Russia." Also on Tuesday, Sen. Roy Blount (R-MO) called for the Senate Intelligence Committee, which he serves on, to perform an exhaustive investigation.