A federal judge ruled the National Security Agency's bulk collection of data on nearly every call made by U.S. phone subscribers is likely unconstitutional, providing ammunition to efforts to rein in surveillance programs that have expanded since the September 11, 2001, terrorist attacks.
Here are five questions about the implications of the court's ruling:
Q: How does the ruling affect the NSA's data collection?
A: The NSA continues to have legal authority, based on Section 215 of the Patriot Act, to collect phone metadata, which includes numbers called and length of calls.
The judge stayed his ruling to give the government time to appeal. But the ruling provides the first indication yet that the courts could put some limits on surveillance that the political branches of government -- Congress and successive administrations under Republican and Democratic presidents -- have not done.
If the ruling stands, it also means that more Americans could sue to have their data removed from the NSA's data collection programs.
Q: Why does the government collect the data and has it been useful for national security?
A: The Obama administration and officials from the Bush administration say the program is crucial to protecting national security.
NSA Director Keith Alexander has said the program, in conjunction with others, has thwarted more than 50 attacks. Supporters of the program in Congress put the number of attacks prevented at closer to 12 or 13.
In Monday's ruling, U.S. District Judge Richard Leon said the program doesn't appear useful at all, adding that government had not cited "a single instance" in which analysis of the NSA phone data program stopped an imminent terror attack.
Q: What have other courts said about the NSA program?
A: Until Monday's ruling, 15 different judges of the Foreign Intelligence Surveillance Court had ruled 35 times that the NSA's phone data collection is legal under Section 215 of the Patriot Act, the government says.
The legality rests also in part on a 1979 Supreme Court ruling, called Smith v Maryland, that phone subscriber rights to privacy didn't include information held by the phone company, such as raw data on phone calls.
Last month, a federal judge in California rejected an attempt to challenge the NSA program, saying the expectation of privacy doesn't extend to phone data.
The Supreme Court, in a separate case last year, required police to seek warrants to attach a GPS device to a suspect's car, but most justices questioned whether the 1979 precedent still was valid given the advances in technology.
Q: What does the ruling mean for Edward Snowden, who disclosed the program?
A: The ruling could increase public support for the idea that Snowden deserves amnesty for exposing a program that now at least one judge thinks is unconstitutional. But the Obama administration flatly rejects any talk of amnesty.
The administration has said there is no plan to seek the death penalty if Snowden returns from his temporary exile in Russia to face federal charges for leaking NSA classified documents.
But Attorney General Eric Holder, in an interview with CNN, said of Snowden: "I think that he has clearly broken the law and harmed the nation that he claims to have loved."
Q: Are there efforts to change the NSA programs?
A: There are multiple proposals to change the NSA data collection programs, but the ones that have the likelihood of passing Congress would still allow the data collection to continue.
Some proposals would take the phone data program out of the NSA's hands and require the phone companies to collect the data, then allow the government to seek access to it. NSA officials call it a "hot potato" that no one wants to hold onto.