Federal authorities prosecuting brothers on allegations they plotted to blow up a high-profile target in New York City are refusing to confirm publicly whether they cracked the case by employing a style of warrantless electronic eavesdropping first introduced by President George W. Bush in the wake of 9/11.

The reason, attorneys affiliated with the defense suggest, is because such a concession would create the nation's first eligible defendant to challenge the constitutionality of the surveillance tactics, which Congress codified into law in 2008 and then again in December.

The government has never publicly conceded it has used evidence in a criminal case obtained through the National Security Agency's post-9/11 mass surveillance program. A single acknowledgment could open the floodgates to challenge the surveillance tactic, which Supreme Court Justice Stephen Breyer in February noted that "commonsense" (.pdf) tells us is being employed by federal investigators.

The terrorism case concerns brothers Raees Alam Qazi, 20, and Sheheryar Alam Qazi, 30. Among other things, prosecutors said the younger Pakistani-born brother surfed Al-Qaida internet sites to learn how to build a bomb. The FBI recorded telephone calls linking him to a plot to blow up a New York landmark last year.

Prosecutors on Thursday told the Florida federal judge presiding over the case that evidence stemmed from a warrant targeting the defendants issued by a closed-door tribunal known as the Foreign Intelligence Surveillance Court. The court was adopted in the wake of the President Richard M. Nixon Watergate scandal, and provided authorities a way to conduct authorized surveillance in secret while under the auspices of a court for the purpose of acquiring "foreign intelligence."

However, the law creating the court was greatly expanded in 2008 and again in 2012, under what is known as the FISA Amendments Act.

The act, which even a member of Congress said is constitutionally suspect, generally requires the Foreign Intelligence Surveillance Court to rubber-stamp terror-related electronic surveillance requests. The government does not have to identify the target or facility to be monitored. It can begin surveillance a week before making the request, and the surveillance can continue during the appeals process if, in a rare case, the secret FISA court rejects the surveillance application.

U.S. Magistrate John O'Sullivan, the judge in the brothers' case, said the law allows for "the mass acquisition of U.S. citizens' and residents' international communications without individualized judicial oversight or supervision." It's the same form of surveillance the Bush administration conceded it began employing days after the September 11, 2001 terror attacks.

Magistrate O'Sullivan, agreeing with a defense motion, ordered (.pdf) prosecutors last week to say whether the government first acquired evidence against the indicted brothers (.pdf) using the Bush-style surveillance, and then used that evidence to obtain the traditional warrant from the secret court.

The path the authorities chose is relevant for a host of reasons.

Among them, the government has never publicly admitted in a prosecution that it employed warrantless surveillance under the FISA Amendments Act. Doing so likely would trigger legal challenges over whether the tactic is constitutional – and would threaten the Qazi brothers' case and perhaps countless others.

"This could open the door again at the Supreme Court," said Patrick Toomey, national security fellow at the American Civil Liberties Union.

The development comes nearly three months after a fractured Supreme Court halted a legal challenge to the warrantless surveillance law at issue in the brothers' case.

A divided Supreme Court, ruling 5-4, set aside the challenge because the plaintiffs – journalists and human-rights groups – had no evidence they were surveilled under FISA Amendment Act authority. Justice Samuel Alito, writing for the majority, said the plaintiffs "merely speculate and make assumptions."

But the court's decision did not foreclose a constitutional challenge. The justices said that, if the government "intends to use" evidence obtained in such a manner, the defendant "may challenge the lawfulness of the acquisition."

Justice Breyer, in dissent, said the case should have proceeded to trial. Of the spying, he wrote: "Indeed it is as likely to take place as are most future events that commonsense inference and ordinary knowledge of human nature tell us will happen."

In court documents Thursday, Assistant U.S. Attorney Karen E. Gilbert urged O'Sullivan to set aside his order in the Qazi matter.

Gilbert said that, within weeks, the authorities will file a declaration from U.S. Attorney General Eric Holder that the information O'Sullivan has ordered disclosed is "sensitive national security information." (.pdf)