Declan McCullagh/CNET

A group of prominent tech companies argues that US government gag orders that prohibit them from disclosing what type of national security information requests they receive are a violation of their free-speech rights.

In court documents (PDF) unsealed Friday, Google, Facebook, Microsoft, and Yahoo contend that the gag orders, called national security letters, or NSLs, are a "prohibition on speech [that] violates the First Amendment."

"The government has sought to participate in public debate over its use of the NSL statute," the companies wrote in a brief filed with the 9th US Circuit Court of Appeals in April. "It should not be permitted to gag those best suited to offer an informed viewpoint in that debate; the parties that have received NSLs."

NSLs are secret requests to Web and telecommunications companies requesting the "name, address, length of service," and other account information about users that's relevant to a national security investigation. No court approval is required for the electronic data-gathering technique, and disclosing the existence of the FBI's secret requests is not permitted.

The companies have sought legal permission for greater transparency about the government requests since last summer when reports based on documents leaked by former NSA contractor Edward Snowden alleged that they provided the NSA with "direct access" to their servers through a so-called PRISM program. The companies have denied that allegation and petitioned the government to allow them to publish, in detail, the types of national security requests they have received under the controversial Foreign Intelligence Surveillance Act.

The companies say they do not want to disclose information related to a specific NSL that might jeopardize an investigation, but rather "more detailed aggregate statistics about the volume, scope, and type of NSLs that the government uses to demand information about their users."

The US government has responded that the companies have no First Amendment right to disclose information gathered from participation in a secret government investigation, according to the filing.

The companies argue that there is no precedent to suggest that disclosure of the government requests is within any traditionally unprotected category of free speech.

"The government attempts to sidestep the serious First Amendment issues raised in this case by arguing that there is no First Amendment right to disclose information gained from participation in a secret government investigation," the companies said. "That is incorrect."

CNET has contacted the US Justice Department for comment and will update this report when we learn more