The FBI raided the office and home of Michael D. Cohen, President Trump's personal lawyer, on Monday. The move enraged the president, who called the raid a "disgrace," and has captivated the attention of both political parties.
A former U.S. attorney told the Washington Post that the raids were like "dropping a bomb on Trump's front porch."
Here's what you should know about the investigation and the people involved.
Cohen has been an ardent defender in Trump's business, personal, and political affairs for more than a decade.
He gained headlines recently when he acknowledged that he had paid porn actress Stormy Daniels $130,000 — just days before the 2016 presidential election — to stay silent about her alleged affair with Trump. Daniels has said the affair happened in 2006. (Trump told reporters last week that he did not know about the payment.)
Agents were seeking records on payments to Daniels and another woman who said she had an affair with Trump, according to the New York Times. The newspaper identified the second woman as Karen McDougal, an ex-Playboy model who said she had an affair with Trump shortly after the birth of his son in 2006. McDougal was paid $150,000 by American Media Inc., the National Enquirer's parent company, whose chief executive is a friend of Trump, the Times said.
Several former officials at the Federal Election Commission have said the payment to Daniels appeared to be a violation of campaign finance laws, and multiple Washington-based groups have filed complaints with the FEC, urging it to investigate.
The Times has said Deputy Attorney General Rod Rosenstein, a Philadelphia native and Trump appointee, authorized the raids.
Rosenstein last year appointed special counsel Robert S. Mueller III to investigate potential ties between Russia and the Trump campaign during the presidential election.
Monday's raids were part of an investigation that Mueller referred to federal prosecutors in New York, according to the Washington Post. The Post said Cohen is under federal investigation over possible bank fraud, wire fraud, and campaign-finance violations.
Under Justice Department regulations, Mueller must consult with Rosenstein when his investigators uncover new evidence that may fall outside his original mandate. Rosenstein then determines whether to allow Mueller to proceed or to assign the matter to another U.S. attorney or another part of the Justice Department.
As he has before, Trump on Tuesday called the investigation into his associates a "witch hunt."
On Tuesday afternoon, White House press secretary Sarah Huckabee Sanders said Trump believed he has the power to fire Mueller — a move that Senate Judiciary Committee Chairman Charles E. Grassley (R., Iowa) warned would be "suicide."
This isn't the first time Mueller has potentially been on the chopping block. Trump last summer ordered Mueller be fired, then backed down after the White House counsel refused to carry out the order, the New York Times reported in January.