The US Supreme Court on Monday will take up a case involving three Muslim men in California who say they were surveilled at their mosque by the FBI after the September 11, 2001 attacks based solely on their religion.

The three men -- Yassir Fazaga, imam of the Orange County Islamic Foundation, along with Ali Uddin Malik and Yasser Abdelrahim -- say the FBI sent a confidential informant to several mosques in the county in 2006 and 2007, ordering the man to pose as a convert and gather information.

"The FBI employed a paid informant person with a prior criminal history to infiltrate these mosques," said Ahilan Arulanantham, a lawyer with the ACLU civil rights group who will represent the plaintiffs before the high court.

The informant, he said, "told everybody that he was a convert, that he was wanting to rediscover his French-Algerian roots."

"He then was instructed by the FBI to gather as much information as possible on people in this community -- cell phones, email addresses, conversations, which he secretly recorded," Arulanantham told reporters.

The ACLU says the informant recorded religious prayer groups in the mosque, leaving a secret recording device hidden in his car key fob, as well as secretly making videos in mosques, homes and businesses.

Arulanantham said the informant "started, again at his FBI handlers' behalf, to try to incite violence, but he scared a bunch of people, when he was talking about things like bombing, Jihad and the war in Iraq and Afghanistan (and) they reported him to the FBI."

The lawyer said the informant then "became disgruntled," argued with his FBI handlers, and ultimately decided to go public with his experience.

The imam and his two congregants then filed a complaint against the FBI for spying on them in violation of federal law and their constitutional rights.

Religious discrimination  

The Justice Department has maintained that it launched the surveillance program for objective reasons -- not because those being watched were Muslims.

But it has invoked an official state secrets act in refusing to say what the alleged reasons were and it called on the courts to reject the case.

A district court in California dismissed the plaintiffs' claim, accepting the FBI argument that state secrets risked being revealed.

But the US Ninth Circuit Court of Appeals disagreed, saying the lower court should have held closed-door hearings to evaluate any secret evidence.

The FBI appealed that decision, and the US high court agreed to hear the case.

It will decide whether a district court can consider classified evidence in determining whether secret government surveillance is lawful.

The case is "tremendously important," Arulanantham said, because it goes to the question of whether the government can simply invoke state secrets to quash any challenge to its surveillance programs -- even in the face of "very serious claims, well supported by declarations, of religious discrimination."

The case is Federal Bureau of Investigation v. Fazaga, and the court is expected to render its decision by June 2022.