Retired Adm. Michael Rogers, former director of the National Security Agency, has been cooperating with the Justice Department’s probe into the origins of the counterintelligence investigation of the Trump presidential campaign’s alleged ties to Russia, according to four people familiar with Rogers’s participation.

Rogers has met the prosecutor leading the probe, Connecticut U.S. Attorney John Durham, on multiple occasions, according to two people familiar with Rogers’s cooperation. While the substance of those meetings is not clear, Rogers has cooperated voluntarily, several people with knowledge of the matter said.

Rogers, who retired in May 2018, did not respond to requests for comment.

The inquiry has been a pillar of Attorney General William Barr’s tenure. He appointed Durham to lead the inquiry last spring, directing him to determine whether the FBI was justified in opening a counterintelligence investigation into Russian meddling in the 2016 presidential election and alleged links between Russia and the Trump campaign, among other matters. What began as a broad review has turned into a criminal investigation, according to the New York Times. Barr has described the use of undercover FBI agents to investigate members of the campaign as “spying.”

Last week, a separate, nonpartisan review of the investigation by the Justice Department inspector general concluded that while the FBI and Justice Department committed serious errors in their applications to surveil former Trump campaign adviser Carter Page, the investigation was opened properly and without political bias. Barr and Durham took the unusual step of publicly disagreeing with some of the inspector general’s conclusions, with Barr describing the FBI’s justification for the inquiry as “very flimsy.”

Rogers’s voluntary participation, which has not been previously reported, makes him the first former intelligence director known to have been interviewed for the probe.

“He’s been very cooperative,” one former intelligence officer who has knowledge of Rogers’s meetings with the Justice Department said.

Politico and NBC News have previously reported that Durham intends to interview both former CIA Director John Brennan and former Director of National Intelligence James Clapper. It is unclear if that has happened. Brennan and the Justice Department declined to comment. Clapper could not be reached for comment.

The Times reported on Thursday that Durham is examining Brennan’s congressional testimony and communications with a focus on what the former CIA director may have told other officials about his views on the so-called Steele dossier, a set of unverified allegations about links between Russia, Trump, and his campaign compiled by former British intelligence officer Christopher Steele.

Rogers is no stranger to the controversy surrounding the 2016 election. Shortly after Trump won the presidency, Rogers traveled to Trump Tower in New York, where he provided an unsolicited briefing to the then president-elect. Rogers informed Trump that the NSA knew that the Russians interfered in the election, according to three people familiar with the briefing. Despite delivering what Rogers told a confidant was “bad news,” Trump would keep Rogers on as NSA director while dismissing Brennan and Clapper.

In January 2017 just before Trump took office, the intelligence community released an unclassified assessment concluding that Russia interfered in the election. The assessment was based on a combination of intelligence collected and reviewed by the NSA, CIA, and FBI.

Russia’s initial purpose, the assessment found, was to undermine confidence in American democracy, but the effort ultimately focused on damaging Hillary Clinton’s campaign in an effort to help elect Trump. While all three intelligence agencies agreed on that aspect of the assessment, the CIA and FBI expressed “high confidence” that the Russian government sought to help Trump win “by discrediting Secretary Clinton and publicly contrasting her unfavorably to him,” while Rogers’s NSA had only “moderate confidence” in that finding.

Trump entered his presidency deeply suspicious of the U.S. intelligence community and skeptical of the assessment. He has spent much of his administration claiming that he is the victim of a “deep-state” coup, beginning with the counterintelligence investigation into his presidential campaign. He has downplayed the intelligence community’s conclusions about Russia’s responsibility for hacking the Democratic National Committee computer system and providing internal emails to WikiLeaks, which published them beginning in July 2016, instead affirming conspiracy theories that blame Ukraine for stealing the emails.

A year into the Trump administration, in February 2018, Rogers testified at a Senate hearing that the White House had given the NSA no orders or instructions for countering further Russian election meddling.

“President Putin has clearly come to the conclusion that there’s little price to pay and that therefore ‘I can continue this activity,’” Rogers said. “Clearly, what we have done is not enough.”

Four months later in Helsinki, Trump said that he confronted the Russian president about meddling in the election. But Vladimir Putin denied that his government was involved, and Trump said he believed him, directly contradicting Rogers and the other U.S. intelligence directors.

Rogers was concerned that his testimony before Congress drew the president’s ire, according to a former Trump White House official who spoke with Rogers earlier this year.

“He asked if the president was mad at him,” the former official said. “I told him, ‘No way, the president has always liked you.’”

The White House declined to comment.

Durham’s inquiry into the origins of the Russia probe has perpetuated the bitter partisan conflict fueled earlier by special counsel Robert Mueller’s investigation. Among Mueller’s key findings was that Russia’s military intelligence unit, the GRU, stole Clinton campaign manager John Podesta’s emails, along with emails from the DNC, and delivered them to WikiLeaks. The Mueller investigation led to federal indictments or guilty pleas from 34 people and three companies, but concluded that there was insufficient evidence to charge anyone in the Trump campaign with coordinating with the Russian government.

Yet the Mueller probe, the recent inspector general’s report, and now the Durham investigation have done little to bridge the yawning political divide between Trump and his supporters, who continue to see him as the victim of a politically motivated “witch hunt,” and career intelligence and national security officials, who view the Durham investigation as an effort to punish those who led U.S. efforts to investigate Russia’s election meddling. In May, Trump gave Barr the unprecedented authority to review and declassify intelligence related to the Russia investigation, further inflaming national security veterans.

Durham’s investigation has also sought information from foreign governments. This summer, Barr and Durham traveled to Italy to request information from Italian intelligence officials about Joseph Mifsud, a Maltese professor who first told a Trump campaign adviser that the Russians had “dirt” on Clinton in the form of stolen emails. That claim played a central role in the FBI’s decision to open an investigation into the Trump campaign. But in the conservative press and the right-wing social media ecosystem, Mifsud was portrayed as part of an Obama administration plot to entrap and frame Trump. The inspector general’s report concluded that there is no evidence that Mifsud had any affiliation with the FBI.

Barr’s visit to Italy coincided with Trump’s offer to trade congressionally approved military aid to Ukraine for that country’s help in pursuing the unsupported allegations that Ukraine hacked the DNC and framed Russia. Trump’s efforts to solicit “a favor” from Ukrainian President Volodymyr Zelensky — that Zelensky publicly announce an investigation into purported Ukrainian-backed hacking and look into alleged corruption by Democratic presidential candidate and former Vice President Joseph Biden on behalf of Biden’s son Hunter — led to Trump’s impeachment in the House of Representatives this week.