A sharply-divided U.S. Supreme Court on Tuesday threw out an attempt by American citizens to challenge the expansion of a surveillance law used to monitor conversations of foreign spies and terrorist suspects.

With a 5-4 vote, the high court ruled that a group of American lawyers, journalists and organizations cannot sue to challenge the 2008 expansion of the Foreign Intelligence Surveillance Act (FISA) because they can't prove that the government will monitor their conversations along with those of potential foreign terrorist and intelligence targets.

The outcome was the first in the current Supreme Court term to divide along ideological lines, with the conservative justices prevailing.

Justices "have been reluctant to endorse standing theories that require guesswork," said Justice Samuel Alito, who wrote for the court's majority.

The Foreign Intelligence Surveillance Act, or FISA, was enacted in 1978. It allows the government to monitor conversations of foreign spies and terrorist suspects abroad for intelligence purposes. The 2008 FISA amendments allow the government to obtain from a secret court broad, yearlong intercept orders, raising the prospect that phone calls and emails between those foreign targets and innocent Americans in this country would be swept under the umbrella of surveillance.

Court rules case is only speculative

Without proof that the law would directly affect them, Americans cannot sue, Alito said in the ruling.

Despite their documented fears and the expense of activities that some Americans have taken to be sure they don't get caught up in government monitoring, they "have set forth no specific facts demonstrating that the communications of their foreign contacts will be targeted," he added.

Alito also said the FISA expansion merely authorizes, but does not mandate or direct, the government monitoring. Because of that, he said, "respondents' allegations are necessarily conjectural. Simply put, respondents can only speculate as to how the attorney general and the Director of National Intelligence will exercise their discretion in determining which communications to target."

Alito was joined in his decision by Chief Justice John Roberts and Justices Anthony Kennedy, Antonin Scalia and Clarence Thomas.

'High probability' plaintiffs are affected

Justice Stephen Breyer, writing in dissent, said that he would have allowed the lawsuit to move forward because he thinks "the government has a strong motive to listen to conversations of the kind described."

"We need only assume that the government is doing its job (to find out about, and combat terrorism) in order to conclude that there is a high probability that the government will intercept at least some electronic communication to which at least some of the plaintiffs are party," Breyer said. "The majority is wrong when it describes the harm threatened plaintiffs as "speculative," Breyer said.

'"The decision effectively insulates the government's increasingly broad surveillance powers from meaningful court review, threatening constitutional liberties in the name of secrecy and security'—Jonathan Hafetz, national security expert

He was joined in his dissent by Justices Ruth Bader Ginsburg, Sonia Sotomayor and Elena Kagan.

Jonathan Hafetz, an expert on national security and privacy issues who teaches at Seton Hall University's law school, said, "The decision effectively insulates the government's increasingly broad surveillance powers from meaningful court review, threatening constitutional liberties in the name of secrecy and security." Hafetz used to work for the American Civil Liberties Union, which represented the plaintiffs in the lawsuit.

A federal judge originally threw out the lawsuit, saying the plaintiffs lacked standing to sue. But the 2nd U.S. Circuit Court of Appeals reinstated the lawsuit. The Supreme Court was not considering the constitutionality of the expansion, only whether lawyers could file a lawsuit to challenge it in federal court.

Alito re-emphasized that point, saying the decision did not insulate the FISA expansion from judicial review, and he suggested a couple of ways a challenge could be brought to court, including a scenario in which an American lawyer actually did get swept up in FISA monitoring.

"It is possible that the monitoring of the target's conversations with his or her attorney would provide grounds for a claim of standing on the part of the attorney," Alito said. "Such an attorney would certainly have a stronger evidentiary basis for establishing standing than do respondents in the present case."