House Intelligence Committee Chairman Devin Nunes (R-Calif.) enters a secure area in the Capitol where his panel has interviewed witnesses in the Russia investigation. (J. Scott Applewhite/AP)

At the center of the firestorm over a congressional memo that President Trump and his allies say reveals federal authorities' missteps is a 40-year-old law passed in the wake of explosive domestic spying scandals.

The Foreign Intelligence Surveillance Act (FISA) grew from congressional investigations into spy agencies' eavesdropping on Americans, including civil rights activists and protesters against the Vietnam War, without warrants.

The law created a warrant requirement for federal authorities to intercept the communications of anyone in the United States, including foreigners. It says the Justice Department must prove to a federal judge that there is probable cause to believe any American target is an agent of a foreign power and engaged in acts such as spying on behalf of a foreign government.

Now, the House Intelligence Committee's Republican leadership, which plans to release the memo, suggests a politicized FBI and Justice Department abused the law to put surveillance on a former Trump campaign aide to advance an investigation into Russian meddling in the 2016 election.

"It's possible, but not likely," said Elizabeth Goitein, co-director of the liberty and national security program at the Brennan Center for Justice. "The warrant was obtained under the part of FISA that is least susceptible to abuse."

Goitein, who has been highly critical of U.S. surveillance practices, won't say it's impossible. "Abuses happen in all areas of government," she said. But the warrant in question was approved in 2016 — when the FISA court "denied many times more applications than any year before," she noted.

Lost in the noise are some of the fundamentals about the surveillance law. FISA has several parts, but the one at issue now is the least controversial.

Title I of FISA governs intercepts of phone calls and emails, and it requires a detailed paper trail, with evidence presented to a federal judge that is first reviewed at several levels of the Justice Department.

Other sections of the law have generated major debates — over the collection without warrants of Americans' phone data and the search of Americans' emails and phone calls within a trove of communications targeting foreigners but collected without individual warrants.

Before FISA, the process for conducting national security wiretaps was much looser, with no requirement that a judge approve the surveillance.

"FISA changed all that," said Morton Halperin, a longtime expert on foreign policy and civil liberties who helped draft the law when he was Washington director of the American Civil Liberties Union.

The memo in question was drafted by the staff of House Intelligence Committee Chairman Devin Nunes (R-Calif.). It suggests the FBI and Justice Department abused their authority under FISA to obtain a wiretap order on a Trump associate, Carter Page.

Though the president has lobbed charges that the FBI surveilled his presidential campaign in 2016, the warrant to spy on Page was obtained in October 2016 — several weeks after Page left the campaign. Still, the application would get careful scrutiny, current and former officials said.

Nunes's memo suggests the Justice Department secured the surveillance order without informing the judge that the source of some of the material was Democratic-funded research.

Who financed theresearch may have had little bearing on the judge's decision to approve the surveillance.

When government lawyers ask for a warrant based on information from a confidential source, they inform the judge of any relevant facts, including bias, that might cast doubt on the source's credibility, said David Kris, a leading FISA expert who served as the assistant attorney general for national security in the Obama administration. "But there is no requirement for elaborate accounting," Kris said. "Courts routinely accept and uphold affidavits that generally describe a source's shortcomings — whether they be prior convictions, drug use, false or inconsistent statements, membership in a criminal enterprise, direct involvement in the defendant's misconduct, or outright hatred of the defendant — without including every specific detail."

Republicans are using a little-known provision of House rules to release their memo despite FBI concerns. The provision was part of 1976 legislation that established the Senate Intelligence Committee, an outgrowth of congressional investigations into domestic surveillance abuses. It was incorporated into the law that created the House Intelligence panel and is now part of the House rules.

The provision permits the committee to disclose classified information following established procedures, if "the public interest would be served by such disclosure" and unless the president certifies that a "threat to the national interest" outweighs the public interest.

On Wednesday, in a rare unsigned statement, the FBI stated it had "grave concerns" about the memo's release, citing "material omissions of fact that fundamentally impact" its accuracy.

"That the FBI has grave concerns should give one pause but should not be dispositive," said Halperin, who drafted the original provision. "The important thing is we want the Congress to be able to challenge executive branch determinations that things not be made public."

Halperin, who is no supporter of the House GOP, lauded the effort to use the provision, and said Congress should use it more often.

On Monday, the House panel voted along party lines to release the memo. In another vote, the Republican majority essentially blocked the public release of a Democratic memo that seeks to counter the GOP document.

Goitein, a former aide to former senator Russ Feingold (D-Wis.), said the disclosure provision exists for good reason. "Congress needs to be able to bring information to public light when the executive branch is improperly trying to conceal it."

But, she said, "if the goal here were to make sure the American public was aware of the facts surrounding this FISA application, then the Republicans would have voted to release the Democratic memo as well."

Shane Harris contributed to this report.