Several Supreme Court justices expressed concern Monday that lawyers, journalists and human rights researchers who think they are caught up in a sweeping federal surveillance law may never be able to challenge the law in court.

Those contesting Congress’s 2008 amendments to the Foreign Intelligence Surveillance Act (FISA) say they are in a Catch-22: The government says they cannot challenge the law unless they can show that their communications are under surveillance, but such information is strictly secret.

The amendments were meant to strengthen the government’s monitoring of potential terrorists and allow it to obtain permission from a secret court to conduct surveillance on communications between those in the United States and targets overseas.

Jameel Jaffer of the American Civil Liberties Union said that means that lawyers, journalists and others who are likely to be in communication with people overseas who are of interest to the government will be caught in what he called “dragnet surveillance.”

His clients must alter their normal activities, he said: being more circumspect in telephone conversations and e-mails, and in some cases traveling abroad to meet with clients or sources.

An analysis of the 2011-2012 Supreme Court session, including justice voting patterns and key cases.

A district court dismissed their standing to bring the lawsuit, but the U.S. Court of Appeals for the 2nd Circuit said it could proceed.

Solicitor General Donald B. Verrilli Jr. told the justices that it is not enough for those who want to challenge the law to speculate that they may be affected by it.

“The government conduct being challenged has to either have occurred or be ‘certainly impending,’ ” Verrilli said, quoting from one of the court’s precedents.

That prompted a chorus of questions from the court’s liberals about whether anyone would ever be able to challenge the law. The targets overseas have no constitutional rights, and their U.S. lawyers are not technically the subjects of the wiretaps, they said.

“I assume that it is an injury for an American speaking in America to have his communication intercepted against his will by the American government,” Justice Stephen G. Breyer said. “We take that as a harm; is that right?”

Verrilli answered: “It may be a harm, yes.”

“Okay,” Breyer responded. “So the question is, how likely is that to occur?”

Verrilli stuck to his argument that those challenging the law could only speculate that their communications would be overheard, prompting a long round of questioning from Justice Elena Kagan, who preceded him in the job as the government’s top appellate lawyer.

“As a lawyer, would you take precautions, or would you pick up the phone and start writing e-mails to all those people?” Kagan asked.

Verrilli said lawyers had reason to be cautious under the previous FISA law, or under ethics requirements, but not necessarily the 2008 amendments.

Justice Antonin Scalia, Verrilli’s tormentor during last term’s arguments over the Affordable Care Act, was his ally Monday. Scalia said it might be true that no one is in a position to challenge the law — but so what?

“We’ve had cases in the past where it is clear that nobody would have standing to challenge what is brought before this court,” Scalia said. “And we’ve said that that just proves that under our system of separated powers, it is none of our business.”

He noted that the government’s plans for surveillance are reviewed by the secret FISA court, which is also charged with looking out for the constitutional rights of Americans.

Chief Justice John G. Roberts Jr. told Jaffer that he faced a high hurdle in establishing that his clients could be so certain of government action that they would have standing to challenge the law.

“Our standard is ‘certainly impending,’ and you articulated it by saying ‘substantial risk,’ ” Roberts said. “There is obviously a vast difference between those two.”

It appeared from the questioning that Justice Anthony M. Kennedy might hold the balance in the case. He indicated to Verrilli that it seemed more than speculation that the lawyers with clients overseas had good reason to think the government was listening.

“It’s hard for me to think that the government isn’t using all of the powers at its command under the law in order to protect this country,” Kennedy said.

Verrilli attempted to stress the purpose of the anti-terrorism law.

“They are asking you to invalidate a vitally important national security statute based not on a concrete application,” but only speculation, he said.

But for one thing, Justice Ruth Bader Ginsburg said, “there may be dozens of concrete applications affecting the plaintiffs in this case, but we will never know.”

And for another, Kagan said, the question at this point is not on the merits of the law. “The question is only: Can they make their argument to a court?”

The case is Clapper v. Amnesty International .