A federal appeals court this week will review whether the government can secretly conduct electronic surveillance on Americans without first obtaining a warrant.

The case, to be brought before a panel of the U.S. Court of Appeals for the 9th Circuit on Wednesday, could have sweeping digital-privacy implications, and it represents one of the most direct challenges to the legal authority for government spying in the post-Snowden era. Many observers expect the case to ultimately reach the Supreme Court.

At issue is whether the FBI can use so-called national security letters, or NSLs, to compel companies to hand over communications data or financial records of certain users for the purposes of a national security investigation. These letters permit the FBI to collect telephone and Internet data of suspects without court approval and they often place a gag order on companies, which prevents them from disclosing the government order.

National security letters have been around since the late 1970s but have grown in importance and frequency in recent years. Hundreds of thousands of such letters have been issued since the post-9/11 USA Patriot Act expanded their authority, and an overwhelming majority have been accompanied by gag orders. The Justice Department argues that NSLs are necessary to protect national security and thwart terrorist attacks.

But in 2011, an unidentified telecommunications company, represented by open-Internet activists with the Electronic Frontier Foundation, challenged the authority of an NSL it received, as well as the gag order preventing public disclosure.

Last year, the case landed in front of U.S. District Judge Susan Illston, who ruled that the FBI's use of NSLs represented an unconstitutional breach of the First Amendment. She found the limited, after-the-fact judicial oversight of NSLs insufficient and ordered the government to cease using them and, additionally, to halt enforcement of their gag orders.

But Illston allowed the government 90 days to appeal, and because of "significant constitutional and national security issues at stake," enforcement of her ruling was stayed.

Illston's opinion, which was seen as a startling rebuke of intelligence agencies' surveillance powers, came months before former National Security Agency contractor Edward Snowden leaked a trove of top-secret documents revealing the size and scope of several of the government's surveillance programs. The Snowden revelations have broadened the significance of this case, privacy hawks argue.

"From the standpoint of vindicating our First Amendment rights and establishing a principle that there are limits to what governments can do, [this case] is extremely important," said Andrew Crocker, a legal fellow with EFF. "You can't deny that the public eyes [are] on national security investigations and how the government uses its legal authorities to collect information on Americans."

The case has also attracted attention from some privacy hawks on Capitol Hill, where a yearlong effort to reform the government's surveillance practices remains in limbo. In a friend-of-the-court brief, Reps. Zoe Lofgren, Thomas Massie, Jared Polis, and Anna Eshoo warned that NSLs "are profoundly problematic because the FBI has extraordinary discretion to issue these demands unilaterally and shroud them in secrecy."

The lawmakers additionally noted that the Justice Department's own inspector general has "documented widespread misuse" of NSLs, which they say are so secret they are difficult to monitor.

"Because Congress must depend on information reported by the FBI to conduct oversight, and NSL recipients are barred from disclosing even the most basic information about these demands, it is exceedingly difficult to evaluate the Bureau's use of this controversial power," the lawmakers wrote.

Crocker said that most of what the public knows about NSLs comes from inspector general reports, which can often be opaque and heavily redacted. One such report released by the Justice Department in August found that the FBI had unintentionally spied on the data of some Americans who were not the intended targets of investigations because of routine typographical errors.

"We found that the FBI's corrective measures have not completely eliminated potential intelligence violations resulting from typographical errors in the identification of a telephone number, email address, or Social Security number in an NSL," the report read. It also noted that the FBI had fulfilled 23 of 28 prior recommendations for improving the agency's use of NSLs.

In the wake of Snowden's leaks, the government has attempted to appease some of the concerns surrounding its surveillance programs, with mixed results. In January, shortly after President Obama pledged to reform the NSA's spying apparatus, Attorney General Eric Holder and Director of National Intelligence James Clapper announced a policy change to allow companies to reveal more details about the number of national security orders they receive.

But those relaxed restraints, which allow for reporting on the quantity of NSLs received every six months, but only within bands of 1,000, have been roundly criticized by tech companies and transparency advocates as not going far enough. (Even if a company received zero requests for data via NSLs, for example, the disclosure could only read 0 to 999.)

On Tuesday, Twitter announced it filed a suit in federal court seeking to publish the full details of its semi-annual transparency report.

"It's our belief that we are entitled under the First Amendment to respond to our users' concerns and to the statements of U.S. government officials by providing information about the scope of U.S. government surveillance—including what types of legal process have not been received," wrote Ben Lee, Twitter's vice president of legal affairs, in a blog post. "We should be free to do this in a meaningful way, rather than in broad, inexact ranges."

Wednesday's oral arguments are before a three-judge panel of the 9th Circuit Court of Appeals in San Francisco. The court is often regarded as the most liberal of the appeals circuits in the United States.

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