The Washington Post
reported that the FBI and Justice Department obtained the warrant on suspicions that Page knowingly engaged in secret intelligence activities on behalf of Russia. Page told CNN the FISA warrant was "unjustified."
So how exactly does the court work? And why does it have a controversial reputation? Here's what you need to know:
The FISA court is a tribunal established in 1978 that decides whether to approve wiretaps, data collection and government requests to monitor suspected terrorists and spies.
It's just blocks away from the White House and Capitol, inside a secure area of the US District Court on Constitution Avenue. But it's completely out of the public eye.
Officials won't say exactly where the FISA courtroom is inside the bunker-like complex. It's so secretive, the room is tightly sealed to prevent eavesdropping.
How does the court work?
Eleven federal district judges
serve on a rotating basis, usually for one week at a time
. All judges have a maximum term of seven years with the FISA court.
And only one person has the power to appoint the judges: US Chief Justice John Roberts
Congress doesn't need to confirm which federal judges take on the added responsibility of serving on the FISA court.
The judges come from across the country -- in fact, they have to come from at least seven of the US judicial circuits.
In theory, those judges can issue warrants from anywhere, said University of Texas law professor and CNN contributor Steve Vladeck. But in practice, the judges usually issue orders during the weeks when they're "on rotation" at the FISA court in Washington.
Can a president authorize a wiretap, as Trump has claimed?
In a Saturday morning tweetstorm last month, Trump accused his predecessor, Barack Obama
, of wiretapping the phones at Trump Tower before the election. He
called Obama a "Bad (or sick) guy!"
But the Trump administration has not given any evidence to support the claim that Obama ordered a wiretap of his phones.
In fact, a president cannot authorize a wiretap. A president can -- through investigative agencies -- request a warrant, but only a court or judge can order one to be issued.
A federal judge could only approve a warrant to wiretap Trump's phones under very limited circumstances, Vladeck said.
One such circumstance would be if the judge found probable cause that Trump committed a federal crime. Another would be if a judge found probable cause to believe that individuals whom Trump or his associates were speaking to were agents of a foreign power.
But a former senior US official with direct knowledge of the Justice Department's investigations said investigators never sought a warrant to monitor Trump's phones.
And former Director of National Intelligence James Clapper told NBC News
he "can deny" there was a FISA court order to wiretap Trump.
"For the part of the national security apparatus that I oversaw as DNI, there was no such wiretap activity mounted against the President, the president-elect at the time, or as a candidate, or against his campaign," Clapper said last month.
What was the original goal of the FISA court?
It started back in 1978 with the court's namesake, the Foreign Intelligence Surveillance Act. That law was enacted "to authorize electronic surveillance to obtain foreign intelligence information."
Back then, the Cold War was in full force. And foreign spying -- not terrorism -- was the big concern.
FISA also came in the wake of the Watergate scandal
and after revelations that the government had been using national security as a pretext to spy on citizens, such as the FBI's spying on the Rev. Martin Luther King Jr
The FISA court started by granting individual warrants for collecting certain pieces of electronic data. But big changes came in the 21st century.
After the 9/11 terror attacks, the court started authorizing more sweeping collections of mass data.
In 2008, for example, changes in surveillance laws gave the attorney general and the national intelligence director more authority to order "mass acquisition" of electronic traffic as long as it's related to a terror or espionage investigation.
In other words, a FISA court judge could authorize the collection of a telecom company's entire database of phone records if it's deemed relevant to counterterrorism efforts.
Why has it come under criticism?
Controversy over the FISA court's broad powers blew up in 2013 when Edward Snowden revealed a secret court order approving the mass collection of metadata
from telecom giant Verizon and Internet companies such as Apple, Facebook, Google, Microsoft and Yahoo.
That led to a heated debate about the limits of privacy and due process.
But there are other reasons the FISA court has come under fire.
Because it's closed off from the public and only hears the federal government's side, some say the court basically "rubber-stamps
" any request from the government.
The FISA court does routinely send applications back to the government to be modified and narrowed, Vladeck said. But the vast majority of applications eventually get approved.
Between 1979 and 2015, virtually all requests for surveillance were approved by the FISA court, though some were modified, according to the Electronic Privacy Information Center
, a privacy watchdog group.
And a 2016 Justice Department report
showed that of the 1,457 requests made to the FISA court in 2015 for permission to conduct electronic surveillance, one was withdrawn by the government. As for the rest, "FISC did not deny any applications in whole, or in part," the Justice Department said.