But the warrant cuts both ways for Trump. Applying for one suggests federal investigators had strong suspicions about Page’s ties to Russian officials. And to obtain the warrant, the Justice Department would have persuaded a federal judge serving the Foreign Intelligence Surveillance Court—often simply referred to as the FISA court—that investigators had probable cause to believe a Trump campaign official was either acting as an agent for a foreign power or knowingly aiding one.

Congress created the FISA court in 1978 to place judicial restraints on American intelligence services after a series of domestic spying scandals in the early 1970s. The court is unlike any other tribunal in the American legal system: Its proceedings are secret and non-adversarial, with only Justice Department lawyers presenting evidence to a single federal judge. The judges themselves are selected by Chief Justice John Roberts from among the ranks of the federal judiciary.

The court’s secretive nature has long drawn skepticism from civil-liberties advocates, especially after Edward Snowden’s mass-surveillance revelations in 2013. Among the documents Snowden released to the public was a FISA warrant authorizing the FBI to obtain all telephone metadata from every U.S. customer of a Verizon subsidiary. The disclosure prompted lawsuits by the ACLU asking federal courts to block the surveillance order.

According to the Post, part of the FBI’s warrant application relied on Page’s reported contacts with a Russian intelligence official in 2013, contact Page himself recently acknowledged to BuzzFeed News. It’s not clear if Page knew he was speaking to an intelligence official at the time.

The Post also reported that application’s collection of Page’s contact with Russian officials included ones that haven’t been publicly disclosed before. FISA warrants typically last for 90 days but can be renewed; the FISA court reportedly renewed the warrant targeting Page “at least once,” the Post reported. It’s not clear whether the FBI’s surveillance of him is still ongoing.

Page joined the Trump campaign’s national-security team in March. Campaign officials have since downplayed his role, which effectively ended in September as reports of Page’s ties to Russian officials began to circulate in the press. But his presence proved controversial even before then. His perceived pro-Russian stances drew scorn from the wider foreign-policy establishment, which also generally shunned Trump before he secured the Republican nomination.

In one memorable episode last July, Page, with the Trump campaign’s assent, traveled to Moscow to give a commencement address at the New Economic School. He denied meeting with any Russian officials under sanctions by the U.S. government at the time.

Page is one of multiple Trump advisors whose ties to Russia came under intense scrutiny during and after the 2016 election. In January, U.S. intelligence agencies publicly concluded that the Russian government used cyberthefts against Democratic Party organizations and Hillary Clinton’s inner circle to undermine her presidential campaign for President Trump’s benefit. Trump has denied that he or his campaign played any role in the disclosures, which he frequently cited on the campaign trail as proof of Clinton’s untrustworthiness.