Flynn resigned early Monday following revelations that he misled administration officials regarding his communications with the Russian ambassador. This put him in a vulnerable position as he was prone to blackmail.
"I inadvertently briefed the Vice President-elect and others with incomplete information regarding my phone calls with the Russian ambassador.
I have sincerely apologized to the President and the Vice President, and they have accepted my apology," Flynn wrote in part of his resignation letter.
Gen. Keith Kellog will hold the fort in an acting capacity, with his most recent posting being that of National Security Council chief of staff. He is also a front-runner to take up the position alongside retired Gen. David Petraeus and former Vice Admiral Bob Harward according to a senior administration official.
According to the source, the issue with Flynn was not about if he talked to the Russians but if he told the truth to Pence. It has been established that Flynn did not intend to mislead the Vice President, but the conversation with the Russians could have slipped his mind.
"Not remembering is not a quality we can have for the national security adviser," the source said.
Another source revealed Trump had to decide to 'hang on' in there when it came to Flynn, but information continued to flood in and that is when it became apparent Flynn had to resign. Asked how the President took it, the source said, "He's moving on."