"No call or meeting w/Russian ambassador. Ever," McCaskill tweeted. "Ambassadors call members of Foreign [Relations Committee]."
Soon after that tweet, it was revealed she did interact with the Russian ambassador.
And now, CNN has learned, McCaskill spent an evening at a black-tie reception at the ambassador's Washington residence in November 2015.
McCaskill was photographed at the event, honoring former Democratic Rep. James Symington, who hails from her state of Missouri and worked to promote US-Russia relations.
In an interview, McCaskill acknowledged attending the dinner, but she said she only did so because of her long-standing relationship with Symington, whom she said "kind of got me started in politics." She claimed the 140-character limit on Twitter did not let her clarify that she never met "one-on-one" with the Russian ambassador, and added she "did not" speak with Kislyak at the reception.
McCaskill, who is one of the most vulnerable Democrats up for reelection next year, conceded she should have exercised more caution in her initial tweet.
"I should've been careful about the 140 characters and given it context," McCaskill said. "But it's not the first or the last time my tweets will get me in trouble."
A spokesman for McCaskill later said the senator interned for Symington in college and considers him a mentor.
Kislyak's meetings with Trump officials during the election season have become a major source of controversy for the White House -- not just because of the meetings themselves, but also because a number of prominent officials have failed to disclose the meetings until they have become public.
Indeed, the controversy over McCaskill's comments started once it was revealed that Attorney General Jeff Sessions failed to disclose during his Senate confirmation proceedings the fact that he had multiple meetings with Kislyak during the election year. The meetings raised suspicions given Russia's role in trying to sway the elections and Sessions' role as a high-profile Trump surrogate.
But Sessions said the meetings were not campaign related, arguing they occurred in his capacity as an Alabama senator who was a member of the Senate Armed Services Committee.
To make her point that Sessions' meetings were unusual, McCaskill fired off the March tweet making the case that members of that committee rarely -- if ever -- meet with the Russian ambassador.
After her March tweet, it was revealed that McCaskill tweeted twice about interacting with the Russian ambassador: once in 2013 about a meeting pertaining to US adoptions and again in 2015 about a phone call with the ambassador about the Iran nuclear deal.
Asked about the discrepancy at the time, McCaskill claimed that her meeting with the Russian ambassador was part of a larger meeting with a group of senators and that her phone call was a brief one about the nuclear deal.
And in an interview last week with CNN, she also made a similar distinction in explaining her presence at the Russian ambassador's home in 2015.
"So on those two occasions, I was in the presence of the Russian ambassador, was in the same place I was, but never did he come to my office, never did he request a meeting with me, never did I have a meeting with him as a member of the Armed Services Committee," McCaskill said. "140 characters were my enemy there because Jeff Sessions was giving the impression that these were part of his -- part and parcel of his duty on the Armed Services Committee."