President Trump's former campaign chairman Paul Manafort, Manafort's former business associate Rick Gates and Trump campaign adviser George Papadopoulos have all been charged in the special counsel's investigation into Russian election interference. (Jenny Starrs/The Washington Post)

This post, originally published in August after news the FBI raided Manafort's home, has been updated with news that Manafort and an associate has been charged in a 12-count indictment with conspiracy against the United States, conspiracy to launder money and making false statements.

If the special counsel independent investigation into Russia collusion was going to charge someone five months in, it makes sense they might zero in on Paul Manafort.

In many ways, Trump's former campaign chairman 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.

Manafort and his former business partner, Rick Gates, have been ordered to turn themselves in on charges related to the investigation on Monday. Here's why Manafort has become such a likely target for the special counsel.

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. He likely has knowledge of the inner workings of the campaign, said Jeffrey Jacobovitz, a white collar lawyer who has represented members of the Clinton administration.

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, Jacobovitz and other legal experts said at the time.

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. He's also being investigated for money laundering, a $3.5 million mortgage he took out on his home in the Hamptons and whether he violated any laws by not fully disclosing his work as a foreign agent in Ukraine.

That's significant leverage investigators have on Manafort. If they can't persuade Manafort to cooperate on the Russia investigation — and the FBI raid, then these charges are evidence they couldn't — they could potentially force him to cooperate by threatening him with unrelated legal trouble.

Threatening someone with legal trouble is a common tactic to try to get them to cooperate with the broader investigation, 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. Investigators have also zeroed in on Flynn's son's work abroad.

“It's a first step and it could be one of many,” Jacobovitz said.

For all these reasons, Manafort is also the perfect target to send a message to the rest of Washington,said Jack Sharman, a white-collar lawyer in Alabama and former special counsel for Congress during the Bill Clinton Whitewater investigation who talked to The Fix in August after the FBI raided Manafort's home, unannounced, before dawn. And that message is: the special counsel investigation means business, and isn't afraid to target high level members of the Trump campaign.