We know this not from Mr. Flynn or the administration, but from accounts first provided to The Washington Post by nine current and former government officials who had access to reports from American intelligence and law enforcement agencies that routinely monitor the communications of Russian diplomats. Bizarrely, Mr. Trump told reporters on Friday afternoon that he was unaware of the Post report, but would “look into that.”

By consorting with the Kremlin after it interfered in the election, Mr. Flynn may have violated the Logan Act, which prohibits citizens from negotiating with foreign governments in disputes involving the American government. Prosecution seems unlikely since the act, which dates back to 1799, has never been used. Former American officials may have a point when they say that aggressive enforcement could discourage sensible foreign contact. But there is little doubt that Mr. Flynn displayed the kind of bad judgment that makes him unfit for high office and raises fresh questions about why he kowtows to President Vladimir Putin of Russia.

The episode has also showed that Mr. Flynn has utter disregard for the truth. On Wednesday, he twice told The Post “no” when asked if he discussed sanctions with Mr. Kislyak. But the next day, a spokesman said Mr. Flynn “couldn’t be certain that the topic never came up.”

Mr. Flynn may also have lied to Vice President Mike Pence, who last month told CBS News that Mr. Flynn had not discussed sanctions with Mr. Kislyak, but had an informal chat in which he extended Christmas wishes. Given his background, shouldn’t Mr. Flynn have known that someone would be listening to his conversation and that any falsehoods would be discovered? He was always a flawed choice for national security adviser and is irreparably damaged now. No one can believe what he says.