The constitutionality of the infamous national security letter (NSL) will be further called into question Wednesday by a federal appeals court in the case National Security Letter, Under Seal v. Holder.

Since 9/11 and the Patriot Act, national security letters have become a staple tactic used by the FBI to force companies to hand over customer information under the guise of fighting terrorism and espionage -- a "linchpin of the federal domestic surveillance program, even more so than the NSA's data collection," said Mercury News, who estimated that between 16,511 and 56,507 such letters were issued a year between 2003-2011.

NSLs allow for an intelligence agency to legally request any and all customer data held by companies without first obtaining a warrant, and the letter prohibits recipients from disclosing that they have received the information request in the first place, effectively gagging the company under threat of jail time.

Companies like Google, Yahoo, Microsoft and Facebook say NSLs infringe on free speech rights, and each company has been engaged in a longstanding battle with the government over NSL constitutionality.

One unnamed telecom recipient challenged the legality in court, and last year, San Francisco U.S. District Judge Susan Illston found that NSLs do, in fact, violate the First Amendment because they forbid the recipients from speaking to anyone about them.

NSLs "suffer from significant Constitutional infirmities," Illston said.

The telecom company recipient is prohibited from disclosing itself, as are its lawyers from the Electronic Frontier Foundation.

UCLA law professor Eugene Volokh, who filed a friend-of-the-court brief on behalf of the Internet Archive, which has also received a NSL, said, "The gag order says you not only have to turn over the information, but you can't complain about it."

Yet the Justice Department maintains that such secrecy is necessary when investigating terrorism and espionage, and told the 9th Circuit, "Public disclosure of actions by the government to investigate terrorism and espionage may allow individuals and groups ... to take steps to evade detection, destroy evidence, mislead investigators, conceal future terrorist and foreign intelligence activities and speed plans for an attack."