Weeks into their new roles in President Donald Trump's administration, Attorney General Jeff Sessions and former national security adviser Michael Flynn have faced criticism -- or worse -- for their associations with one person: Russian Ambassador Sergey Kislyak.
Who is this man at the center of the controversy?
Kislyak, 66, has served as Moscow's ambassador to the United States since 2008. The career diplomat is known for his expertise on arms control and has staunchly defended Russia's involvement in Ukraine and Crimea in recent years, and from 1998 to 2003, he was Russia's ambassador to Belgium and NATO.
Since his arrival in the United States, it hasn't been unusual for Kislyak to host large dinners -- or attend them -- to make new contacts and push the Russian agenda.
"I admired the fact that he was trying to reach deep into our government to cultivate relations with all kinds of people," former U.S. Ambassador to Russia Michael McFaul told The New York Times. "I was impressed by the way he went about that kind of socializing, the way he went about entertaining, but always with a political objective."
But it's those interactions that now have him in the spotlight.
He first earned attention in February when The Washington Post reported Flynn talked with Kislyak about U.S. sanctions against Russia prior to Trump's inauguration and the official start of Flynn's job as national security adviser -- a possible violation of federal law.
Those sanctions, imposed by the Obama administration in December, were in response to allegations Russia interfered with the U.S. presidential election. The Trump administration forced Flynn to resign for not being forthright with Vice President Mike Pence about the conversation with Kislyak, though Trump maintained Flynn did nothing wrong by reaching out to foreign officials.
And this week, Kislyak's meetings with Attorney General Jeff Sessions and Trump's son-in-law and senior adviser, Jared Kushner, during the campaign have come under scrutiny. Then-Sen. Sessions met twice with Kislyak, saying it was in his capacity as a senior member of the Armed Service Committee, not as a Trump surrogate.
Sessions on Thursday recused himself from any investigations into Russia's alleged meddling in the 2016 election, though, after backlash from Democratic and Republican lawmakers. Some said he should have revealed the conversations with Kislyak when asked if he had any communications with Russia during the campaign.
CNN reported this week that some U.S. intelligence members consider Kislyak to be a top spy and spy recruiter for Russia. But Russian officials say he's just doing his job as a diplomat by reaching out to and talking to current and soon-to-be members of the U.S. government.
Russian Foreign Ministry spokeswoman Maria Zakharova described him as a "world-class diplomat."
"He was deputy minister of foreign affairs in Russia, who has communicated with American colleagues for decades in different fields, and CNN accused him of being a Russian spy ... of recruiting? Oh my God!" she said in a tense exchange with a CNN reporter.
"I will reveal a military secret: Diplomats are working, and their work consists of making contacts in the host nation," Zakharova later said, according to Sputnik.
Kislyak, too, has defended his interactions with Trump surrogates.
"Our job is to talk to all the people, be it Republicans, be it Democrats, whether they work for a campaign, whether they don't work for a campaign," he said during an appearance at Stanford University in November. "Our job is to understand."
During that appearance, Kislyak expressed regret that U.S.-Russian relations have deteriorated.
"We are living in the worst point in our relations after the end of the Cold War," he said, adding that the United States is attempting to control Russia through sanctions.
It now could be politically harmful to U.S. officials to meet with Kislyak in any capacity, says Paul Saunders, of Center for the National Interest, a conservative think tank.
"Very few people probably would want to meet with Russia's ambassador," Saunders told NPR. "That really doesn't really have anything to do with him personally. But I think it has become, in our political climate, increasingly damaging to admit to contacts like that."
That may not matter, though. Russian Deputy Foreign Minister Anatoly Antonov may be in line to replace Kislyak as Moscow's top diplomat in Washington pending approval by Russia's parliament.