A federal appeals court has declared illegal the National Security Agency program that collects data on the landline calling records of nearly every American. The ruling Thursday, the first of its kind by an appeals court, comes as Congress considers whether to continue, end or overhaul the program before June 1, when the legal provisions authorizing it expire.

Five things to know about the court ruling, the program, and the congressional debate about where to go from here:

NOT-SO-SECRET SURVEILLANCE PROGRAM

At issue is an NSA program that for years has been collecting and storing data on American phone calls — a closely held secret until it was leaked by former NSA systems administrator Edward Snowden in 2013. The NSA collects information on the number called and date and time of the call, and stores it in a database that it queries using phone numbers associated with terrorists overseas. Officials say they don't use the data for any other purpose.

The idea is to hunt for hidden domestic terrorists akin to the hijackers who carried out the 9/11 attacks. But the program has not been particularly valuable as a counter-terrorism tool, and is becoming less so, since, for technical and bureaucratic reasons, the NSA has not been gathering the data on most mobile calls.

DEMOCRATIC APPOINTEES AGREE

A three-judge panel of the 2nd U.S. Circuit Court of Appeals in Manhattan ruled that the practice was not legally justified under the law its creators cited to implement it, Section 215 of the USA Patriot Act. In a unanimous ruling written by Judge Gerard Lynch, the court held that Section 215 "does not authorize the telephone metadata program," despite years of secret legal rulings by an intelligence court that it could.

The appeals court rejected an argument that since the law allows the government to seize records relevant to a terrorism investigation, it was sufficient to declare all the country's phone records relevant. The ruling, however, allows the program to continue, since the provisions expire June 1 and Congress is debating their future.

All three of the 2nd Circuit judges are Democratic appointees.