An adviser to Donald Trump during his election campaign has admitted seeking to set up a meeting between the campaign and Russian officials.
George Papadopoulos pleaded guilty this month to having lied to the FBI over when he met go-betweens, an unsealed justice department statement reveals.
He said he had been told the Russians possessed “dirt” on Hillary Clinton.
Former Trump campaign manager Paul Manafort has been charged with tax fraud in an unrelated case.
The 12 charges brought against Mr Manafort and one of his business associates, Rick Gates, include conspiracy to launder money.
They do not relate to Mr Trump’s campaign but to the pair’s Ukrainian business dealings up to 2015.
An investigation headed by special counsel Robert Mueller is looking into any links between Russia and the Trump campaign. Both sides deny any collusion.
How does the Papadopoulos case affect Trump?
It has the potential to damage the US leader because it relates directly to his campaign.
When he was interviewed by the FBI this January, he falsely claimed that he had met two figures with Russian connections before joining the Trump campaign in March 2016. In fact, he met them after joining the campaign.
One was an unnamed Russian woman who, Mr Papadopoulos believed, had connections to Russian government officials.
He admitted seeking to use her connections in an effort to arrange a meeting “between the Campaign and Russian government officials”.
The other person was an unnamed, London-based professor who was said to have “substantial connections to Russian government officials”.
The professor only took an interest in Mr Papadopoulos because of his status within the Trump campaign, the statement says.
Russian “dirt” on Mrs Clinton, in the form of “thousands of emails”, was allegedly mentioned by the professor at a breakfast meeting in a London hotel on or around 26 April 2016.
The professor said he had been informed about the compromising emails when he met senior Russian government officials on a recent trip to Moscow.
Trump aides have said Mr Papadopoulos played a limited role in the campaign and had no access to Mr Trump, the Associated Press news agency reports.
This may be the real blockbuster
By Anthony Zurcher, BBC News, Washington
Robert Mueller, in journalism terms, buried the lede.
Initial headlines on Monday morning were about Paul Manafort’s indictment for money laundering and undisclosed foreign lobbying activities. The real blockbuster, however, may end up being George Papadopoulos’s plea bargain, which was disclosed shortly after the Manafort news came out.
Unlike Mr Manafort, pinched for activities largely conducted before he joined the Trump team, Mr Papadopoulos has admitted to lying to the FBI about contacts he had with Russian nationals while serving as a Trump foreign policy adviser.
Mr Papadopoulos’s indictment document reveals he learned from his connections that Russia claimed to have “dirt” on Democratic candidate Hillary Clinton in the form of “thousands of emails” in a late April 2016 meeting. Damaging emails from the Democratic National Committee and Clinton campaign chair John Podesta started emerging, through Wikileaks, just a few months later.
Now Mr Papadopoulos is co-operating with the independent counsel’s office. At the very least, he has told them who in the top levels of the Trump campaign he was passing this information to.
If Mr Mueller’s case ends up about more than just illegal activities by Mr Manafort and a business associate years ago, Mr Papadopoulos could be a key piece of the puzzle.
What are the charges against Manafort and Gates?
The indictment against the two men looks at their links to pro-Russian politicians in Ukraine between 2006 and 2015.
It says they acted as “unregistered agents” of Ukrainian politician Viktor Yanukovych and his party, both in opposition and government.
Mr Yanukovych was deposed as president in 2014 amid mass unrest over his pro-Russian policies.
Mr Manafort is accused of having laundered more than $18m (£14m) through offshore bank accounts, using it to buy property, goods and services in transactions concealed from the US authorities.
He is said to have “used his hidden overseas wealth to enjoy a lavish lifestyle” in America.
Altogether, at least $75m in payments from Ukraine flowed through the accounts, the indictment says.
Mr Gates is accused of having transferred more than $3m from the offshore accounts to other accounts he controlled.
No immediate comment from lawyers for Mr Manafort and Mr Gates was reported after the charges were revealed.
Responding to news of the charges, Mr Trump tweeted to point out that they did not concern his campaign and asked why “the focus” was not on alleged wrongdoing involving Mrs Clinton instead.
Why did Trump bring up Clinton?
On Friday, Mr Trump accused Mrs Clinton of links with Moscow.
Republican lawmakers have alleged that a uranium deal with a Russian company in 2010, when Mrs Clinton was secretary of state, was sealed in exchange for donations to her husband’s charity.
A Congressional investigation has been opened into the case. Democrats say it is an attempt to divert attention from the alleged ties between Russia and Mr Trump.