Former National Security Adviser Susan Rice has been named by multiple sources as the Obama administation official who "unmasked" Trump campaign or transition team officials who were caught up in surveillance of other targets.

The development has rocked Washington, as critics say it could be evidence that the outgoing administration used its foreign surveillance powers to spy on the incoming White House team.

The law that allows the federal intelligence community to spy on foreign actors, the Foreign Intelligence Surveillance Act, has safeguards in place that are designed to protect Americans who are "incidentally" recorded. One protection is that they must be "masked."

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WHAT IS "UNMASKING?"

When a U.S. intelligence agency, such as the National Security Agency, conducts surveillance of a foreigner inside the U.S., sometimes that surveillance will include the name of an American that the foreigner is speaking to or about. When this happens, intelligence analysts are obliged to hide or "minimize" the name of the American, unless knowing the American's name is necessary to understanding the foreign intelligence described in the report.

"If, for instance, an intelligence piece were about Russian intelligence assets engaged in an operation to influence political figures, the identity of the political figure would be necessary," said Todd Hinnen, head of the Justice Department's National Security Division during the Obama administration and a National Security Council staff member under George W. Bush. "Unless you know the answer to that question, you can't appreciate the meaning and importance of the intelligence."

WHO CAN UNMASK AN AMERICANS NAME?

The answer varies by U.S. intelligence agency. Last month, National Security Agency director Michael Rogers told lawmakers there are 20 people at the NSA, including himself, who have the authority to reveal the name of an American in a surveillance report.

IS IT ILLEGAL TO UNMASK AN AMERICAN'S NAME?

It is not illegal to unmask the identity of an American incidentally swept up in foreign surveillance, but Fox News Senior Judicial Analyst Andrew Napolitano said the unmasking must have a national security rationale.

Unmasking the name or names of Americans for political purposes would be highly improper, Leaking those names to the media would be a crime, according to legal experts. So far, there is no evidence that Rice leaked the names she ordered unmasked, although the resignation of her successor, Lt. Gen. Michael Flynn, amid reports he was caught on surveillance talking to the Russian ambassador, indicates someone did.

The Associated Press contributed to this report