As President Donald Trump himself said, the recently revealed predawn FBI raid of his former campaign chairman’s Alexandria, Virginia home sent a “very, very strong signal.”

That signal, former federal prosecutors told TPM, is that Paul Manafort is the linchpin in special counsel Robert Mueller’s investigation into Russia’s election interference. Whether the investigation ends up centering on potential financial crimes or possible collusion between the Trump campaign and Russia, those former prosecutors said nailing the goods on Manafort will be key to Mueller’s case.

From the millions of dollars Manafort reportedly owed to pro-Russian interests, to his bank accounts in the tax haven of Cyprus, to his attendance at a 2016 meeting billed as an opportunity for the Russian government to provide the Trump campaign with information that would damage Hillary Clinton’s chances in the presidential race, to his retroactive filing with the Justice Department as a foreign agent, the longtime GOP operative seems to have a toe in almost every part of the special counsel’s sprawling probe. And a slew of recent reports indicate Mueller’s team is tightening the vise on the former campaign chairman.

In a surprise move for someone in the thick of a federal investigation, Manafort announced through a spokesman Thursday that he was no longer working with his longtime counsel, WilmerHale, and was “in the process” of retaining a law firm with expertise in international tax cases, Miller & Chevalier.

Michael Zeldin, a former federal prosecutor, served as special counsel to Mueller when he was Assistant Attorney General in the DOJ’s Criminal Division.

He told TPM that if prosecutors saw no evidence of collusion, they could simply compile evidence of potential financial crimes, “put a ribbon” around that part of their investigation, and seek an indictment.

“Or, they’re going to put a ribbon on that and tell him, ‘We’ve found these charges that could send you to jail for a period of time, we would like to know about collusion, or deals,'” he added. “Essentially, ‘We need you to be truthful about this, we don’t think you’ve been truthful, we’re giving you a chance to be truthful now.’”

Though the raid, which Bloomberg reported had come as a surprise to Manafort, was the most striking sign yet that investigators are closing in on the former campaign chairman, they’re pursuing plenty of other paths.

Investigators have sent subpoenas through a Washington, D.C. grand jury to global banks, requesting account information and transaction records for Manafort, some of his companies and his former business partner on political work in Ukraine, Rick Gates, Bloomberg reported Thursday. Gates also worked on the Trump campaign, transition and for a pro-Trump group.

Others close to Manafort have been pulled into the investigation: Manafort’s son-in-law and occasional partner in real estate deals, Jeffrey Yohai, who’s currently being sued for fraud by a former investor, provided documents and information to investigators during a meeting in New York two months ago, CNN reported Thursday. Investigators were seeking his cooperation in the probe against Manafort, two sources told CNN. Yohai’s attorney, Aaron May, declined TPM’s request for comment.

These multiple pressure points against Manafort indicate to former federal prosecutors that Mueller did not simply conduct the raid to “scare him,” as assistant Watergate prosecutor Nick Akerman put it.

“They wouldn’t put themselves in a position where they couldn’t totally justify it,” Akerman told TPM. “Not just to the judge, but to the public. They’re going to want to be able to say we had no choice but to do this and that the evidence for it was pretty compelling.”

“My sense is that Mueller is acquiring the evidence that he needs to acquire in order for him to proceed with his investigation and that is his primary motive,” Zeldin concurred. “If by acquiring that evidence in the manner that he did, it sends some sort of shockwave through the person who is the target of that evidence retrieval and has him rethink his position, so be it.”

The extent to which Manafort has been forthcoming with investigators is unclear. Confirming that the raid took place, Manafort spokesman Jason Maloni said that the former campaign chairman had “consistently cooperated with law enforcement.” In July, Maloni told Politico that it was “silly” for that cooperation to lead anyone to think Manafort was serving as a cooperating witness; he did not respond to a question from TPM about whether that comment still stands.

A Mueller spokesperson declined comment for this story.

The special counsel investigation had something of a head start in investigating Manafort, as Mueller assumed control of several pre-existing probes into the former campaign chairman’s business activities and record-keeping. He took over an investigation run out of the Southern District of New York into whether Manafort laundered money from eastern Europe, as well as a separate probe into his belated registration under the Foreign Agents Registration Act (FARA) for his political work in Ukraine.

Josh Rosenstein, an attorney at Sandler Reiff Lamb Rosenstein & Birkenstock who specializes in FARA, said that although prosecutions under the act are quite rare, it is a “distinct possibility” that Mueller’s team could pursue one against Manafort.

“When FARA becomes a high profile issue as certainly it has been here in and of itself, the risk of prosecution is certainly much higher,” Rosenstein said, noting that FARA charges would likely be brought as a secondary charge to any potential financial crimes.

Other Trump associates touched by the investigation are surely watching Manafort’s situation closely. One of the President’s personal attorneys, John Dowd, sent a furious email to the Wall Street Journal in the wee hours of Thursday morning, complaining that the raid on Manafort’s home represented an “extraordinary invasion of privacy” and “gross abuse of the judicial process” done only for “shock value” (Zeldin, Mueller’s former special counsel, noted that Dowd had “no standing to complain” because he doesn’t represent Manafort).

Asked about the raid while on vacation at his Bedminster, New Jersey golf club, Trump distanced himself from his former campaign chairman, calling him a “decent man” but telling reporters “[I] haven’t spoken to him for a long time.” In the mildest show of support, the President questioned whether the FBI needed to wake Manafort so early in the morning to search his home.

“I thought it was pretty tough stuff to wake him up, perhaps his family was there,” Trump mulled. “I think that’s pretty tough stuff.”

Correction: The original story misidentified the location of the home raided by FBI agents. It was in Alexandria, Virginia, not Arlington.