What is the FISA court, and why is it so secretive?

(CNN) It's a highly fortified courtroom where some of the most secretive government decisions get made. And you've probably never heard of it.

But in the coming days, the Foreign Intelligence Surveillance Court -- known as the FISA court or FISC -- will get more attention after President Donald Trump claimed he was wiretapped during the Obama administration. (Trump has not offered any evidence supporting his claim.)

So how exactly does the court work? And why does it have a controversial reputation? Here's what you need to know:

What is the FISA court?

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 located 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: Chief Justice of the United States 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 claimed?

No. 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, said CNN contributor and University of Texas law professor Steve Vladeck.

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 that 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.

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.

What changed?

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.

The FISA court does hear challenges, though. In 2013, Yahoo scored a win when FISC ruled that the government must publish court papers from 2008 detailing Yahoo's objections to releasing users' data without a warrant.

But there are other reasons the FISA court has come under fire.

Because it's closed off from the public and only hears the 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.