Congress has just days to extend, reform, or repeal FISA - the controversial law that lets the US government track calls and emails

Francisco Seco/AP

The National Security Agency has implanted software in nearly 100,000 computers around the world - but not in the United States - that allows the US to conduct surveillance on those machines.

• A controversial surveillance law is set to expire at the end of the month.
• But Congress is still bitterly divided over whether to reauthorize it, reform it, or completely repeal it.
• FISA Section 702, which allows the US government to conduct warrantless communications collections, was born out of the post-9/11 surveillance era.

Congress has just days to extend, reform, or repeal Section 702 of the Foreign Intelligence Surveillance Act (FISA), a highly controversial surveillance law that allows the US government to track and collect the communications of foreigners overseas without a warrant, even if Americans' communications are incidentally picked up along the way.

Lawmakers have until they leave for the holiday break on Friday to reauthorize Section 702 before it expires on December 31, but they're still divided over how to address it.

While US officials say it's integral to national security, citing its use in disrupting a terrorist plot to blow up the New York City subway system in 2009, privacy rights advocates worry the law oversteps its bounds and infringes on Americans' Fourth amendment rights.

For example, when foreigners subject to US surveillance are in contact with people in the US, the government can incidentally pick up the communications of those Americans through what is called "backdoor" collection.

Edward Snowden's leaks of classified NSA documents in 2013 revealed widespread abuse of this kind of collection. The leaks showed that the government had been using the 702 provision to sweep up vast amounts of Americans' data directly from the servers of major Internet companies like Microsoft, Google, and Facebook through the legal authority of a surveillance program called PRISM.

The Electronic Frontier Foundation, a privacy rights group, argues on its website that the Snowden revelations show that "the fact that Section 702 surveillance regularly results in the collection and search of innocent Americans' communications is an intended and inherent part of the system."

But the US government defends its use of Section 702 and maintains that it does not target Americans or any other people located inside the country.

More than four years since Snowden's leaks, the debate over the government's spying authority rages on.

Here's how we got here and what lawmakers are doing about it: