WASHINGTON — In a black Monday for Donald Trump's White House, the special counsel investigating possible coordination between the Kremlin and the Trump presidential campaign announced the first charges, indicting Trump's former campaign chairman and revealing how an adviser lied to the FBI about meetings with Russian intermediaries.

Paul Manafort, who steered Trump's campaign for much of last year, and business associate Rick Gates ended the day under house arrest on charges that they funneled payments through foreign companies and bank accounts as part of their private political work in Ukraine.

George Papadopoulos, also a former campaign adviser, faced further questioning and then sentencing in the first — and so far only — criminal case that links the Trump election effort to the Kremlin.

Manafort and Gates, who pleaded not guilty in federal court, are not charged with any wrongdoing as part of the Trump campaign, and the president immediately sought to distance himself from the allegations. He said on Twitter that the alleged crimes occurred “years ago,” and he insisted anew there was “NO COLLUSION” between his campaign and Russia.

But potentially more perilous for the president was the guilty plea by former adviser Papadopoulos, who admitted in newly unsealed court papers that he was told in April 2016 that the Russians had “dirt” on Democratic rival Clinton in the form of “thousands of emails,” well before it became public that the Democratic National Committee and Clinton campaign chairman John Podesta's emails had been hacked.

Papadopoulos was not charged with having improper communications with Russians but rather with lying to FBI agents.

The developments, including the unexpected unsealing of a guilty plea, usher Mueller's investigation into a new, more serious phase. And the revelations in the guilty plea about an adviser's Russian contacts could complicate the president's assertions that his campaign had never coordinated with the Russian government to tip the 2016 presidential election in his favor, the central issue behind Mueller's mandate.