As special counsel Robert Mueller reportedly works to determine whether the President attempted to obstruct justice with regard to the sprawling federal probe into Russia’s interference in the 2016 election, he need look no further than the words of Donald Trump himself.

Trump’s tweets (“Phony collusion with the Russians story”) and public comments (firing his FBI director because of “the Russia thing”) provide a wealth of evidence for the special counsel as he probes why and how the President may have tried to make the Russia investigation disappear.

Mueller “already has the proof that Trump fired Comey because of the Russia investigation,” according to Cornell Law School professor Jens Ohlin.

“Trump just flat-out said it on national television,” he said. “So what would normally be the most difficult part of the investigation is not difficult at all. The whole world has the evidence.”

It’s rare for the reported target of an obstruction of justice investigation to freely provide so much insight into his motives. But even setting aside Trump’s public statements, Mueller would have a staggeringly long list of known fact witnesses and potential fact witnesses to question.

As the Washington Post revealed this week, the obstruction-of-justice inquiry began just days after James Comey was fired as head of the FBI in early May. Mueller took control of that probe after he was named special counsel, according to the Post.

Based on various officials’ sworn testimony before Congress and what’s been reported about Trump’s deliberations around firing Comey, the list of potential witnesses Mueller could speak with run the gamut from top Justice Department and intelligence officials to White House advisers—even Trump’s golf buddies. Here’s who could have valuable information to offer Mueller.

Star witness James Comey

Two private conversations that Trump had with Comey would be central to Mueller’s investigation. As Comey testified in vivid detail last week, the day after national security adviser Michael Flynn was forced out, Trump asked him to linger behind after an intelligence briefing in the Oval Office and told him he hoped the then-FBI director could let the Flynn probe “go.” In a subsequent phone conversation on March 30, Comey says Trump asked what he could do to “lift the cloud” the ongoing Russia investigation created over his administration.

These requests, combined with Trump’s previous appeal for his “loyalty,” did not feel casual, Comey said.

“I took it as a direction,” he testified. “It is the President of the United States, with me alone, saying ‘I hope this.’ I took it as, this is what he wants me to do.”]

Comey kept detailed contemporaneous memos of the one-on-one conversations he had with Trump, which a friend, Columbia Law School professor Daniel Richman, reportedly has turned over to Mueller’s office already. The former FBI director testified last week that he documented those conversations out of concern that the President would “lie” about their interactions.

He also testified that he briefed a number of high-level FBI staff about the contents of the memos so that they could corroborate his accounts. Those individuals, who could serve as fact witnesses for Mueller, include Comey’s chief of staff, Jim Rybecki; then-FBI deputy director Andrew McCabe; general counsel James A. Baker’ and McCabe’s chief counsel. According to Comey, some of those debriefing conversations also included David Bowdich, the FBI’s associate deputy director, and Carl Ghattas, the executive assistant director for the national security branch.

Top intelligence officials

Mueller plans to interview a number of senior intelligence officials, including Director of National Intelligence Dan Coats, National Security Agency head Adm. Mike Rogers, and Rogers’ former deputy, Richard Ledgett, as part of the obstruction investigation, according to the Washington Post.

Shortly after Comey first confirmed that the FBI was investigating possible collusion between Trump campaign staffers and Russian operatives, Trump asked Coats and CIA Director Mike Pompeo to stay behind following a March 22 briefing at the White House, according to the Post. Though Coats later testified he never felt pressured to intervene in the Russia investigation, he reportedly told associates at the time that the President had asked if he could convince Comey to lay off an investigation into Flynn.

Days afterward, Trump reportedly called each Coats and Rogers individually to ask that they publicly deny any evidence of collusion—a request they denied.

Ledgett wrote an internal memo documenting Trump’s call with Rogers, according to the Post and the Wall Street Journal. The Journal noted that during that phone conversation, the President cast doubt on the U.S. intelligence community’s assessment that Russia intervened in the 2016 election.

Attendees at Valentine’s Day briefing

While Trump cornered Comey alone on Feb. 14 to discuss the investigation into Flynn, a number of other senior White House officials can speak to the circumstances surrounding that one-on-one conversation. As Comey testified, he, Vice President Mike Pence, CIA Deputy Director Gina Haspel, Attorney General Jeff Sessions, National Counterterrorism Center Director Nick Rasmussen, Homeland Security Secretary John Kelly, and Jared Kushner, Trump’s son-in-law and senior adviser, were all present for a counterterrorism briefing.

Comey testified that when Trump asked him to stay behind afterwards, Sessions and Kushner dawdled.

“My sense was the attorney general knew he shouldn’t be leaving which is why he was lingering,” he said. “I don’t know Mr. Kushner well but I think he picked up on the same thing.”

Trump’s chief of staff, Reince Priebus, also poked his head into the room at one point, according to Comey, before the President waved him away.

Mueller will likely be interested in asking these individuals whether Trump mentioned the Flynn investigation before, during or after that meeting, as well as asking them if they had any insight into why he asked Comey to speak with him alone, legal experts said.

“The particular story with Sessions and Kushner being sent out has to be confirmed by them,” David Golove, a constitutional law expert at New York University, told TPM.

“He might even want to speak to people who were in physical proximity to those rooms who could testify about what people’s reactions were as they were leaving the room, did anyone say anything when they left the room,” Ohlin said, noting administrative staffers like secretaries or schedulers could provide useful insight.

Anyone involved in deliberations over Comey’s firing

Although White House officials have put forth conflicting explanations for Trump’s abrupt dismissal of Comey, Trump told NBC News’ Lester Holt that he would have ousted Comey no matter what advice he received and that the “Russia thing” was on his mind when he decided to go through with it. Comey testified that he believed the President’s account: that he was fired to “change the way the Russia investigation was being conducted.”

Legal experts said Mueller will likely want to speak with the White House officials who reportedly deliberated with Trump and offered support for Comey’s ouster, particularly Kushner and Pence. Priebus, Trump’s daughter and adviser, Ivanka Trump, White House Counsel Don McGahn, and Trump’s longtime bodyguard-turned-director of Oval Office operations Keith Schiller, who delivered Comey’s termination letter to FBI headquarters, also were among those involved, according to the Post.

Both Sessions and Deputy Attorney General Rod Rosenstein, who Trump tasked with writing memos to lay out the case for firing Comey, could also serve as fact witnesses for Mueller. The administration initially used those memos to justify the President’s move before Trump himself publicly blew up that narrative in the interview with Holt.

NatSec officials who heard Trump call Comey a “nut job”

Trump bragged about firing Comey, who he referred to as a “nut job,” to Russian foreign minister Sergey Lavrov and Russia’s ambassador to the U.S., Sergei Kislyak, in an Oval Office meeting the day after he made the bold move. Comey’s removal, Trump told the top diplomats, lifted “great pressure” on him created by the federal Russia investigation, according to a New York Times report.

The Times’ story was based on a document summarizing the gathering and the White House did not dispute it. Secretary of State Rex Tillerson, National Security Adviser H.R. McMaster, and his deputy, Dina Powell, were all present for the meeting, making them potential fact witnesses.

Other unknown administration officials

The President’s loose lips and the unprecedented leakiness of his administration present another potential problem for Trump, as there is a vast network of White House and administration officials who may be privy to information valuable to Mueller.

These include members of the White House communications team and officials at the DOJ and FBI. The Post spoke to some 30 people for its story on the background of Comey’s firing, indicating just how wide a net could be cast.

A wide network of friends and hangers-on

The final group of potential witnesses includes Trump’s close confidantes from New York City and his various private clubs, who he is known to call up to grouse about his administration’s crisis du jour.

“He frequently calls people who work in the media or friends or former partners of his that he’s relied on for counsel—a kind of eclectic collection of people he considers to be loyal and have good advice,” Ohlin said. “So I’d imagine Mueller would want to speak to them all as well.”

Just this week, Trump ally and Newsmax CEO Chris Ruddy kicked off a firestorm by suggesting that the President was considering firing the man now reported to be overseeing the obstruction of justice investigation: Mueller himself.