FBI agents raided the home of President Trump's former campaign chairman Paul Manafort without warning on July 26 with a search warrant, and seized documents and other records, say people familiar with the special counsel investigation. (The Washington Post)

There are a couple reasons the special counsel's expanding Russia investigation might be so interested in former Trump campaign chairman Paul Manafort that FBI agents showed up at his door before dawn, unannounced, searched his home and seized documents, as The Washington Post reports.

In many ways, Manafort is squarely in the crosshairs of the Russia-Trump collusion investigation: His brief tenure as the head of Trump's campaign happened as concerns about Russia's meddling in the 2016 election were heating up, he's got high-level connections to Russia in his own right, and he's got a whole host of scrutinized financial dealings that could make him a useful tool for investigators seeking cooperation.

He's also the perfect target to send a message to the rest of Washington that the special counsel investigation means business, said Jack Sharman, a white-collar lawyer in Alabama and former special counsel for Congress during the Bill Clinton Whitewater investigation.

"One purpose of such a raid is to bring home to the target the fact that the federal prosecution team is moving forward and is not going to defer to or rely on Congress," he said.

Let's zero in why Manafort was the target the FBI decided to send a message through.

1. He was Trump's campaign chairman for several months in 2016. If Robert S. Mueller III's team is investigating Trump-Russia connections, it is not going to stop at some low-level Trump associate. These investigators are going straight to the top to see if they find evidence of collusion, and for several key months in 2016, Manafort was the top.

2. He was in *that* meeting. When Donald Trump Jr. was told the Russian government was trying to help his father win, and oh by the way, do you want to meet with a Russian lawyer who has dirt on Hillary Clinton, Trump Jr. didn't go alone. He brought along Manafort and Jared Kushner, Trump's son-in-law.

It appears Manafort took detailed notes of that meeting, and those notes could be key evidence if there are any collusion-related charges.

That meeting's circumstances are “as close as you can get to a smoking gun” on whether the Trump campaign colluded with Russia, said Jeffrey Jacobovitz, a white-collar lawyer who represented officials in the Clinton White House.

3. He's got lots of ties to Russia. A Russian aluminum magnate. A pro-Russian former Ukrainian president. A Republican congressman who advocates for close ties between the United States and Russia. A business associate from his time in Ukraine who once served in the Russian army and had dinner with Manafort during the campaign.

Of all the Trump campaign officials, Manafort has the most known connections to Russia. Before he joined Trump's campaign, he was a political consultant in Ukraine, where he helped elect a president backed by Russia. (That president was toppled four years later and fled to safety in Russia.) The Post reported in June that Manafort's consulting firm received $17 million over two years from that president's political party, the Party of Regions.


Of all the Trump campaign officials, Paul Manafort has the most known connections to Russia. (Win McNamee/Getty Images)

During the campaign, the New York Times uncovered ledgers in Ukraine for secret, under-the-table payments to the Party of Regions' allies. Manafort's name was in them, though he denied any wrongdoing. The news eventually led to Manafort's resignation from Trump's campaign, three months after he got elevated to the top job.

4. Investigators could use him as leverage. Manafort's role in the Trump campaign isn't the only aspect of his life under federal investigation. The Wall Street Journal has reported that the special counsel is investigating him for money laundering allegations. NBC has reported federal investigators have subpoenaed records related to a $3.5 million mortgage Manafort took out on his home in the Hamptons. And The Post reports that Justice Department officials are also looking into whether he violated any laws by not fully disclosing his work as a foreign agent in Ukraine. (Manafort retroactively filed as one in June, which is how we know how much money he got paid by Ukrainian politicians.)

Much of that is now under Mueller's umbrella. That's significant leverage investigators have on Manafort. If they can't persuade Manafort to cooperate on the Russia investigation — and this search warrant is evidence that they feel they couldn't — they could potentially force him to cooperate by threatening him with unrelated legal trouble. (Manafort has not been, nor do we have any indication he will be, charged with a crime.)

Snagging a big fish with an unrelated crime is a common tactic used by investigators, Jacobovitz said. It's possible the special counsel is using those same kinds of tools on Michael Flynn, Trump's former national security adviser, who is facing legal questions over his failure to disclose his lobbying work for Turkey.

Flynn has asked for immunity from Congress to testify about what he knows. Manafort has provided documents to multiple House and Senate committees and plans to testify sometime later this year — which raises a whole host of questions about why the FBI felt the need to show up and search his home. Either way, now those documents are in the hands of FBI agents, and the message has been sent: They mean business.