Within Mr. Trump’s inner circle, Mr. Flynn appears to have been the primary interlocutor with the Russian envoy. The two were in contact during the campaign and the transition, Mr. Kislyak and current and former American officials have said. But Mr. Sessions served as the chairman of Mr. Trump’s national security committee — a post Democrats said would have made him a much sought-after figure for officials from many foreign countries.

There is nothing unusual about meetings between presidential campaigns and foreign diplomats. Mr. Kislyak was one of several envoys at the Republican National Convention, where his first meeting with Mr. Sessions, according to the attorney general, was a brief encounter after a panel organized by the Heritage Foundation. Ambassadors also attended the Democratic convention, though it was not clear whether Mr. Kislyak was among them.

“Active embassies here consider it as their assignment to stretch out feelers to presidential hopefuls,” said Peter Wittig, the German ambassador, who met most of the Republican candidates, though not Mr. Trump. “I don’t consider it as something unusual or problematic.”

The trouble in Mr. Sessions’s case is that his meeting came as the nation’s intelligence agencies were concluding that Russia had tried to destabilize the election and help Mr. Trump. Mr. Sessions’s initial lack of disclosure of the meetings with Mr. Kislyak fed suspicions that it was more than run-of-the-mill diplomacy.

The disclosure, first reported by The Washington Post, contradicted forceful and repeated denials from the White House that anyone from the Trump campaign had discussions with the Russians. “I have nothing to do with Russia,” Mr. Trump said at a news conference on Feb. 20. “To the best of my knowledge, no person that I deal with does.”