National Security Advisor Michael Flynn is out of a job, and the media buzz about the reasons why involves two concerns often discussed in security clearance circles: truthfulness and vulnerability to blackmail.

The problem arose from conversations Flynn had with Russian Ambassador Sergey Kislyak in late December about U.S. sanctions before President Donald Trump’s inauguration.

The Obama White House anticipated a tit-for-tat response from Russia after it imposed sanctions and expelled 35 Russian diplomats as punishment for Russia’s hacking attack during the presidential election. Instead, nothing happened. That prompted an investigation, according to MSNBC.

Officials reviewed intelligence reports, and intercepted communications and diplomatic cables to discover that Flynn and Kislyak had communicated via text and phone around the time of the sanctions announcement. The Washington Post and the New York Times report that Flynn told Kislyak not to react to the sanctions; that the Trump administration would take care of things once Trump was sworn in.

Acting Attorney General Sally Yates told the White House in late January “that she believed Michael Flynn had misled senior administration officials about the nature of his communications with the Russian ambassador to the United States, and warned that the national security adviser was potentially vulnerable to Russian blackmail,” the Boston Globe reports.

When questioned about Flynn’s calls to Kislyak, Flynn told Vice President Mike Pence that he had not discussed the sanctions, according to The Washington Post. Pence and other Trump officials repeated this denial on news shows in the following days.

Flynn also denied the discussion to The Post on Feb. 8, but revised his statement the following day by saying via a spokesperson that he couldn’t be certain the topic didn’t arise.

The situation came to a head Feb. 13 and Flynn resigned. Following is an excerpt from his resignation letter:

“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.”

Security clearance lawyer Catie Young says there is a lesson in this situation for everyone who holds a security clearance: At no level is a security clearance holder shielded from the requirement to protect sensitive information.

“In addition to Flynn having potentially mishandled confidential information, so potentially did the sources who leaked information related to this incident to the press,” Young says. “But it just proves that this can happen to anyone, and every single security clearance holder must protect their clearance by protecting the information to which they have access.”

Kellyanne Conway, counselor to the president, told FOX News and MSNBC that ultimately, this came down to Flynn’s misleading statements or forgetfulness in characterizing the conversation he had with the Russian ambassador.

“When it comes to security clearance, the importance of being truthful begins with the SF-86,” Young says. “It is vital that an applicant be truthful and candid when providing information on that application, and it continues as long as you hold that clearance.”

Intelligence officials suspended Michael Flynn’s security clearance on Feb. 15, ABC News reports.