National Security Advisor Mike Flynn resigned on Monday after it was revealed he lied about discussing impending sanctions against the Russian government, CNN reports.

Flynn, who served in the job for less than a month, said he had given “incomplete information” to about a telephone call he had with the ambassador in late December about American sanctions against Russia, weeks before Trump’s inauguration. Flynn previously had denied that he had any substantive conversations with Ambassador Sergey I. Kislyak, and Pence repeated that claim in television as recently as earlier this month.

According to the NY Times, Flynn’s resignation letter, which the White House emailed to reporters,he said he had held numerous calls with foreign officials during the transition. “Unfortunately, because of the fast pace of events, I inadvertently briefed the Vice President Elect and others with incomplete information regarding my phone calls with the Russian Ambassador,” he wrote. “I have sincerely apologized to the President and the Vice President, and they have accepted my apology.”

The shakeup now leaves Trump without one of his closest and longest-serving advisers. Flynn had counseled Trump on foreign policy and national security matters since early in the 2016 presidential race.

We will bring you more information as it becomes available.