The U.S. Court of Appeals for the D.C. Circuit vacated the lower court decision from 2013, but in doing so, it did not rule on the merits of the program. Instead, it questioned whether there was clear enough evidence that the plaintiffs had been harmed.
"Plaintiffs must establish a 'substantial likelihood of success on the merits'" wrote Judge Janice Rogers Brown. "Plaintiffs fall short of meeting the higher burden of proof required for a preliminary injunction," she said.
The Court sent the case back down to the lower court for further proceedings.
White House Press Secretary Josh Earnest said the ruling was consistent with the administration's arguments that the program was constitutional.
"However, the President believed that there were important reforms that could be put in place that would both better protect the privacy and civil liberties of the American people, while also making sure that our law enforcement and intelligence officials had the tools that they need to keep us safe," Earnest said at the daily White House briefing.
Alex Abdo, staff attorney with the ACLU, and a critic of the program, emphasized that the ruling was "procedural" and that it did not address the constitutionality or the legality of the NSA surveillance program.
He pointed out that another appeals court
has weighed in on the merits of the program, "and it ruled the government's collection of American's call records was not only unlawful, but 'unprecedented and unwarranted.'"
Some legal experts question the practical impact of Friday's ruling since the lawsuit was filed before Congress changed the law and the 2013 opinion -- issued by Judge Richard J. Leon -- had been stayed pending appeal.
"It doesn't matter for the current state of play because Congress, through the USA FREEDOM Act, fundamentally changed the nature of the metadata program," said Stephen I. Vladeck, a professor of Law at American University Washington College of Law and a CNN contributor.
He said that if the ruling had any broader significance it would be to make it "harder for plaintiffs in general to challenge secret government programs."
Leon reacted swiftly Friday afternoon, scheduling a status conference for next Wednesday.