Former Trump national security adviser Michael Flynn pleaded guilty Friday to lying to the FBI about conversations with Russia’s ambassador and disclosed that he is cooperating with the special counsel’s office.

Flynn is the first person inside President Donald Trump’s administration to be reached by special counsel Robert Mueller’s probe. The developments are a sign that the investigation is intensifying, and details revealed Friday provide the clearest picture yet of coordination between Flynn and other Trump advisers in their contact with Russian officials to influence international policy.

According to an FBI statement, Flynn communicated with then-Russian Ambassador to the US Sergey Kislyak after being asked by a senior Trump transition official to find out how foreign governments stood on a coming UN Security Council resolution about Israel. The prosecutors did not name any transition officials.

Sources familiar with the matter told CNN that Trump’s son-in-law Jared Kushner is the senior official referred to in the statement of offense.

An attorney for Kushner, now a White House senior adviser, did not comment. The White House said late Friday morning that “nothing about the guilty plea or the charge implicates anyone other than Mr. Flynn.

“The conclusion of this phase of the special counsel’s work demonstrates again that the special counsel is moving with all deliberate speed and clears the way for a prompt and reasonable conclusion,” Ty Cobb, a White House lawyer, said in a statement.