Apple submitted a legal brief on Thursday opposing the U.S. government's attempt to force the technology company to unlock an encrypted iPhone belonging to one of the killers in the San Bernardino attack, one day before the filing deadline.

The Federal Bureau of Investigation is seeking Apple's help to access Rizwan Farook's iPhone by disabling some of its passcode protections.

Apple has pushed back, arguing that such a move would set a dangerous precedent and threaten customer security.

"The government's request here creates an unprecedented burden on Apple and violates Apple's First Amendment rights against compelled speech," Apple argued in a copy of the brief provided by the company on Thursday.

Apple warns of slippery slope

The filing represents Apple's first official response since a federal magistrate last Tuesday ordered the company to help the FBI gain access to a phone used by one of the assailants in the Dec. 2, 2015, San Bernardino attack that left 14 people dead and 21 others wounded.

The Justice Department is proposing a "boundless interpretation" of the law that, if left unchecked, could bring disastrous repercussions for digital privacy, the company warned in a memo submitted to Magistrate Sheri Pym.

"The government says: 'Just this once' and 'Just this phone.' But the government knows those statements are not true," lawyers for Apple wrote.

Apple and the FBI2:05

Apple also argued the court order, if upheld, could leave individuals and business vulnerable to an unlimited array of government directives.

"Under the same legal theories advocated by the government here, the government could argue that it should be permitted to force citizens to do all manner of things 'necessary' to assist it in enforcing the laws," Apple said.

It gave examples, "like compelling a pharmaceutical company against its will to produce drugs needed to carry out a lethal injection in furtherance of a lawfully issued death warrant or requiring a journalist to plant a false story in order to help lure out a fugitive."

The filing was made the same day that FBI Director James Comey defended the government's approach during separate appearances on Capitol Hill, telling a congressional panel that court approval of the FBI's request was "unlikely to be a trailblazer" in other cases

While the case "will be instructive for other courts," larger policy questions about reasonable law-enforcement access to encrypted data will likely need to be resolved by Congress and others, Comey said.


Apple's resistance to the FBI has sparked a national debate about whether the government should have technological access — or a 'back door' — to get into privately owned phones (Mark Ralston/AFP/Getty Images)

Tech companies back Apple

Apple's resistance has sparked a national debate about whether the government should have technological access or a "back door" to get into privately owned phones.

The tech giant's court brief builds upon arguments voiced by the company's chief executive Tim Cook and its supporters.

And some of the largest tech companies appear to be lining up behind Apple.

Google and Facebook will both file briefs supporting the iPhone maker, said several sources familiar with the matter who were not authorized to speak publicly about it.

Microsoft will file a friend-of the-court brief as well, company president Brad Smith said in congressional testimony Thursday. The announcement comes just days after Microsoft founder Bill Gates appeared to back the FBI in the dispute.

Twitter also said it will sign a brief in support of Apple.

With files from The Associated Press