In his new report on the Trump-Russia affair, special counsel Robert Mueller revealed that former national security adviser Michael Flynn was already under FBI investigation when he spoke with the Russian ambassador during the transition before President Trump took office. Flynn's conversations with Ambassador Sergey Kislyak were previously thought, by the public at least, to be the reason the FBI became suspicious about, and began investigating, Flynn's conduct.

Flynn was never charged with any crime relating to now-debunked allegations the Trump campaign conspired or coordinated with Russia to fix the 2016 election. Flynn pleaded guilty to lying to the FBI in a Jan. 24, 2017 interview during which he was questioned about the Kislyak calls. Specifically, Flynn denied that he and Kislyak discussed Russia's reaction to new U.S. sanctions over election interference, when a transcript of the conversation, based on a secret wiretap, showed that they had.

Now, Mueller strongly suggests something else was up. Obama administration intelligence officials "were surprised by Russia's decision not to retaliate in response to the sanctions," the report said. "When analyzing Russia's response, they became aware of Flynn's discussion of sanctions with Kislyak. Previously, the FBI had opened an investigation of Flynn based on his relationship with the Russian government. Flynn's contacts with Kislyak became a key component of that investigation." (emphasis added)

Mueller attributed the information, which is on page 26 of Volume II of the report, to interviews with former Justice Department official Mary McCord, who was deeply involved in the Flynn case, and fired FBI Director James Comey.

Flynn, a retired three-star Army general and former director of the Defense Intelligence Agency, had some previous contacts with Russia, most notably in 2015, when he delivered a much-noticed paid speech at an event in Moscow for the Russian television network RT. At the event, Flynn was seated next to Vladimir Putin.

It is not clear whether that would have been enough for the FBI to open an investigation of Flynn "based on his relationship with the Russian government." But there is no doubt that Obama officials were deeply suspicious of Flynn's actions during the transition. Top officials came up with a theory that Flynn had violated the never-before-prosecuted 1799 law known as the Logan Act. FBI investigators "believed that Flynn's calls with Kislyak and subsequent denials about discussing sanctions raised potential Logan Act issues and were relevant to the FBI's broader Russia investigation," the Mueller report said. In addition, Justice Department officials theorized that Flynn's lie to Vice President Mike Pence exposed Flynn to possible Russian blackmail. Together, those theoretical concerns were enough for the FBI to send agents to question Flynn on January 24, 2017. And from that interview came the charges to which Flynn pleaded guilty.

Now comes news that there was an open, Russia-related investigation of Flynn before the Kislyak conversations ever took place. It's unclear what such an investigation was based on, and what it involved. Were there wiretaps? Some other sort of surveillance? Informants? It's all unknown, even after Flynn's guilty plea and long period of cooperation. (Flynn is still awaiting sentencing.)

But Mueller chose to make the existence of the investigation public. Now, the public needs to know more about what is, and was, going on.