WASHINGTON — The Trump administration’s back-to-back controversies over its Russian ties now have at least one thing in common: Ambassador Sergey Kislyak.

Moscow’s top diplomat is a Washington fixture with a sprawling network, and he has emerged as the central figure in the investigations into Trump advisers’ connections with Russia. In a matter of weeks, contact with Kislyak led to the firing of a top adviser to the president and, on Thursday, prompted calls for the Attorney General Jeff Sessions l to resign.

Separately, a White House official confirmed that Trump’s son-in-law Jared Kushner and ousted national security adviser Michael Flynn met with Kislyak at Trump Tower in December for what the official called a brief courtesy meeting. Flynn was pushed out of the White House last month after officials said he misled Vice President Mike Pence about whether he and the ambassador had discussed U.S. sanctions against Russia in a phone call.

At issue Thursday were two meetings between Sessions and Kislyak — one in July and another in September, at the height of concern over Russia’s involvement in hacking of Democratic officials’ emails accounts. Intelligence officials have since concluded that Moscow ordered the hacks to tilt the election toward Trump. In his confirmation hearing, the Alabama Republican denied having contact with any Russian officials, neglecting to mention the meetings with Kislyak, which were first reported by the Washington Post.

The Russian Embassy did not respond to a request for comment.