A sharply-divided Supreme Court on Tuesday threw out an attempt by U.S. 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 can't 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.

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.

Without proof that the law would directly affect them, Americans can't 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.