Two Trump donors have been charged with violations of campaign finance laws, New York prosecutors say.
Businessmen Lev Parnas and Igor Fruman conspired to "funnel foreign money to candidates for federal and state office", say the charges.
The pair have been associated with US President Donald Trump's personal lawyer Rudy Giuliani.
They have been peddling unsubstantiated corruption claims against a top Democratic presidential contender.
Both men are US citizens. Mr Parnas was born in Ukraine, Mr Fruman in Belarus.
The two men were arrested at Dulles International Airport in Virginia before they could board a scheduled flight to Vienna, Austria, on Wednesday night.
They appeared in federal court in Virginia on Thursday.
Their attorney, John Dowd, who previously represented Mr Trump, has not commented to US media.
Two other men, Andrey Kukushkin, a Ukrainian, and David Correia, a US citizen, were also indicted in the same case on Thursday. Mr Correia is not yet in federal custody.
Ukrainian President Volodymyr Zelensky told reporters on Thursday he never met Mr Parnas and Mr Fruman, but "heard about them".
What are they accused of?
Mr Parnas and Mr Fruman have been charged with conspiracy, lying and falsifying records.
The pair are accused of circumventing US campaign finance laws through a money-laundering scheme.
The indictment says they used an unnamed foreign national's money to illegally make political contributions in their own name and lied to the Federal Election Commission.
Mr Parnas and Mr Fruman also allegedly set up an energy company in Florida to conceal the sources of their funding.
They are accused of falsely donating $325,000 (£264,000) in 2018 to a pro-Trump political action committee - thought to be "Committee-1" in the indictment - in the name of this company and claiming it was for company purposes.
This same donation was the subject of a complaint by a campaign finance watchdog.
Together with this donation, Mr Parnas and Mr Fruman gave upwards of $400,000 to Republican campaigns in just a few months.
The contributions allowed the men to meet leaders of the Republican party, including Mr Trump at the White House and at his Florida resort, Mar-a-Lago.
Mr Fruman and Mr Parnas also allegedly raised thousands of dollars in funds for an unnamed congressman in order to seek their "assistance in causing the US government to remove or recall the then-US ambassador to Ukraine".
This was allegedly done in part by "the request of one or more Ukrainian government officials".
Campaign finance records suggest the congressman was former Texas Republican Pete Sessions, who lost his re-election last November, US media report.
The ambassador at the time was Marie Yovanovitch, who is expected to testify before the House of Representatives on Friday about the Ukraine inquiry.
Ms Yovanovitch was dismissed by the Trump administration earlier this year.
A double-headed threat for Trump
The arrest of Lev Parnas and Igor Fruman presents a double-headed threat for Republicans and President Donald Trump.
The first concern is the allegation that the two Soviet-born businessmen were funnelling hundreds of thousands of foreign dollars to Republican groups, state parties and candidates in the middle of the 2016 campaign - an election where foreign meddling had become a very serious concern.
More than a few Republican officeholders and activists are going to have uncomfortable questions about why they were the recipients of foreign largess - and if any strings came with the money. One donation, for example, was allegedly tied to the (eventually successful) effort to remove the US ambassador to Ukraine, Marie Yovanovitch.
That brings up the other source of Republican pain - Mr Parnas and Mr Fruman's close ties to Donald Trump's personal lawyer, Rudy Giuliani, who met with the two the day before they tried to leave the country.
They were already the target of impeachment investigators for their role in helping Giuliani's Ukrainian sleuthing project. Their arrest will only make the Democrats' request for documents and depositions all the more urgent.
There are new threads in the story exposed, and Congress is set to start pulling them.
How is this related to Giuliani?
Mr Parnas has been a longtime associate of Mr Giuliani and, according to the New York Times, enlisted Mr Fruman's help to connect the president's lawyer with Ukranian prosecutors.
The pair helped organise a meeting between former Ukrainian prosecutor Yuri Lutskeno and Mr Giuliani in January, according to the Associated Press.
Mr Giuliani in May referred to Mr Parnas and Mr Fruman as his clients.
In the complaint that sparked the Ukraine inquiry on Capitol Hill, the whistleblower noted concerns that Mr Trump was "using the power of his office to solicit interference from a foreign country" in the upcoming presidential election.
The whistleblower report also repeatedly mentioned Mr Giuliani's involvement in Ukraine - and his work with Mr Parnas and Mr Fruman, who are referred to as his "two associates".
How is this related to Trump?
The Trump administration's role in Ukraine is at the centre of a congressional impeachment inquiry.
The probe concerns Mr Trump's request in a July telephone call for Ukraine's president to launch an investigation into former US Vice-President Joe Biden, now a White House hopeful.
The charges are the first criminal case related to the Ukraine investigation.
These alleged campaign finance violations do not concern Mr Trump directly, though the activities of Mr Parnas and Mr Fruman as two associates of the president's personal lawyer is attracting congressional scrutiny.
The pair were expected to appear on Thursday and Friday before Democratic-led congressional committees investigating Mr Trump.
Following the indictment, lawmakers issued Mr Parnas and Mr Fruman with legal summonses to compel them to submit documents to the inquiry.