Google Inc (GOOG)., operator of the worldâs largest search engine, is challenging a demand by the U.S. government for private user information in a national security probe, according to a court filing.

It âappearsâ to be the first time a major communications company is pushing back after getting a so-called National Security Letter, said the Electronic Frontier Foundation, an Internet privacy group. The challenge comes three weeks after a federal judge in San Francisco ruled that NSLs, which are issued without a warrant, are unconstitutional.

âThe people who are in the best position to challenge the practice are people like Google,â said EFF attorney Matt Zimmerman, who represented an unidentified service provider that won the March 14 ruling. âSo far no one has really stood up for their usersâ among large Internet service providers.

The government has issued 300,000 NSLs since 2000, and only four or five recipients have challenged the letters, Zimmerman said. Civil-rights groups say NSLs give federal agents unchecked powers to spy on people while the government says theyâre a crucial tool in the fight against terrorism and threats to national security.

Google, in its first public disclosure about national security letters, said in a March 5 report that it received in the range of zero to 999 NSLs annually starting in 2009 affecting more than 1,000 accounts. In a company blog post, Richard Salgado, Googleâs legal director for law enforcement and information security, thanked U.S. government officials âfor working with us to provide greater insight into the use of NSLs.â

Google filed a petition to set aside a âlegal processâ pursuant âto 18 U.S.C. Section 3511 (a) and (b),â according to a March 29 filing in federal court in San Francisco seeking a court order to seal its request. Petitions âfiled under Section 3511 of Title 18 to set aside legal process issued under Section 2709 of Title 18 must be filed under seal because Section 2709 prohibits disclosure of the legal process,â Kevan Fornasero, Googleâs lawyer, said in the filing.

The petition itself was lodged under seal with the court. No details of the governmentâs demand for records were disclosed in the filing.

Section 2709 is a federal law authorizing the Federal Bureau of Investigation to issue NSLs requiring wire and electronic communication service providers to turn over subscriber information and other records that the agency certifies are relevant to an investigation of international terrorism or clandestine intelligence activities. The law prohibits NSL recipients from disclosing theyâve received one.

Section 3511 (a) allows recipients of NSLs to petition a federal judge to set aside the request and allows judges to modify or set aside the request if complying with it would be âunreasonable, oppressive or otherwise unlawful.â Section 3511 (b) allows NSL recipients to ask a court to lift the gag order.

U.S. District Judge Susan Illston yesterday granted Googleâs request to seal documents in the case. Illston ruled March 14 that the gag order section of the NSL law was unconstitutional and rendered the entire statute illegal.

Illston said the NSL statutes violated free speech and separation of power principles because the government failed to show that, to protect national security, it needs to always bar people from disclosing the mere fact theyâve received an NSL and the law impermissibly restricted courts from reviewing the need for nondisclosure.

She ordered the FBI to stop issuing NSLs and put her ruling on hold for 90 days to allow the government time to appeal.

âWe are in this interesting in-between moment in which the government is still able to enforce its authority,â said Marc Rotenberg, president and executive director of the Washington- based Electronic Privacy Information Center. âI suspect that this filing is an effort to push the issue further.â

Chris Gaither, a spokesman for Mountain View, California- based Google, declined to comment on the filing. Chris Allen, an FBI spokesman in Washington, also declined to comment.

The case is In Re Google Inc. (GOOG)âs Petition to Set Aside Legal Process, 13-80063, U.S. District Court, Northern District of California (San Francisco).