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In what was considered by some the most important surveillance overhaul in decades, Congress put an end last year to the government’s mass-collection of phone call and other telecommunications data. A series of news reports, the first of which revealed that Verizon was handing over all of its call data to the National Security Agency, culminated in a contentious Senate debate: key portions of the Patriot Act were reauthorized, granting the government broad surveillance powers, but privacy-minded lawmakers won some hard-fought civil liberties protections.

The Senate now faces another countdown clock. This time, over the government’s authority to eavesdrop on foreigners, a policy viewed as a crucial counterterrorism weapon, but one that may entangle innocent Americans and their Constitutional rights.

The FISA Amendments Act (Foreign Intelligence Surveillance Act) was last reauthorized in 2012, and allows the government to collect the electronic communications of foreigners located outside the Unites States. As part of that reauthorization, the surveillance law was set to expire at the end of next year.

Citing privacy concerns and civil liberties violations, critics of FISA point to a provision of the law, known as Section 702, which authorizes the collection of this information through U.S. technology companies. While the government is forbidden from targeting Americans using 702 data collection, the communications of Americans who speak with legally wiretapped foreigners can also be captured.

The extent of 702 surveillance on Americans, and how this information is used by domestic law enforcement, is still unclear — even to members of Congress. The apparent lack of transparency fueled most of the disagreement during a Senate Judiciary Committee hearing Tuesday, and will propel the coming post-Snowden surveillance debate on Capitol Hill.

“Section 702 is an important tool for our national security agencies,” said Sen. Patrick Leahy, the ranking member of the Judiciary Committee. “It is also extremely broad, allowing the government to collect communications without individualized warrants." Leahy proposed adding additional safeguards and transparency requirements before FISA is renewed, including asking for an estimate on the number of Americans who’ve been caught up in the surveillance of foreign targets.

Leahy also took issue with so called “back-door” searches. The searches, according to privacy experts, function as a dubious legal workaround in which U.S. law enforcement surveils a foreign citizen in the hopes of eavesdropping on a particular American with whom the foreigner is associated. “These ‘back-door’ searches raise serious constitutional questions, particularly since the FBI can use them to investigate crimes having nothing to do with national security,” Leahy said.

Elizabeth Goitein, the co-director of the liberty and national security program at New York University Law School, believes section 702 may violate the 4th Amendment, since such end-run searches take place without a particularized warrant. Goitein recommended that an individual court order be required in order for the government to collect communications between a foreign target and an American. She also suggested to lawmakers that they limit FISA’s scope by narrowing the definition of foreign intelligence, the categories of people who can be targeted, and ensuring that notice is given anytime evidence derived from 702 surveillance is presented in court.

Goitein said these changes would help put FISA on better legal ground, and curb some of the data security risks posed by the government amassing a giant database of sensitive information.

For Silicon Valley, these contested surveillance issues matter not only for tech giants operating as stewards of consumer data, but for American businesses facing scrutiny abroad after the Snowden revelations.

The majority of the expert witnesses at the hearing however, told lawmakers that FISA should be renewed without changes, describing 702 as a vital deterrent against pervasive threats of terrorism.

“Section 702 is unquestionable a highly effective source of foreign intelligence,” said Rachel Brand, a member of the Privacy and Civil Liberties Oversight Board, an independent agency formed after 9/11 to review the executive branch’s policy on counterterrorism.

Brand said the board does not recommend any legislative changes to FISA, adding that the government already operates under “strict rules that minimize its privacy impact.” She dismissed the recommendation made by Goitein and her fellow board member, David Medine, that the government first seek a judge’s approval before the 702 database can be used to spy on Americans.

“Requiring additional approval for FBI queries would be a step toward re-erecting the information sharing wall that the government has worked so hard to tear down after 9/11,” she said.

Sen. Dianne Feinstein, who also serves on the secretive Intelligence Committee, seemed to share Brand’s opinion. She expressed her concerns over looming national security dangers and the need for information sharing. “Because I see the intelligence, you get a sense that there is plotting and there is conspiracy going on,” Feinstein said. “We shouldn’t let down our guard. Because to do so is really to invite disaster.” She encouraged renewed skepticism and debate on FISA, but dismissed substantive changes to the law.

“I think that the 702 program is really important and it would be most unfortunate if that program were made ineffective,” Feinstein said.

But in what will likely be a key point of contention, as the deadline to FISA’s reauthorization draws closer, lawmakers including Sens. Leahy and Al Franken will push to learn about the full extent of the surveillance programs. As with last year’s debate of the Patriot Act, disagreement over the need for increased transparency will likely animate negotiations in Washington as Congress aims to set the right balance between national security and fundamental privacy rights.

Said Franken, “The public can’t know if we succeed in striking that balance if they don’t even have the most basic information about our major surveillance programs."

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