Apple filed a formal objection late Tuesday in the tech giant's case involving an FBI request to unlock an iPhone used by one of the San Bernardino shooters.
The document filed in U.S. District Court made good on Apple's promise to fight a judge's Feb 16. order compelling it to assist investigators. Apple "formally objects" to the order out of an "abundance of caution," the filing says.
It was the latest twist in a legal battle that has brought a debate over the balance of privacy and security to the public fore.
The phone in question is the iPhone 5C used by San Bernardino massacre shooter Syed Rizwan Farook, and which was owned by his employer.
The FBI wants Apple's help in disabling an iPhone feature that locks or erases the phone after repeated unsuccessful attempts to enter a password. The bureau has asked Apple to create a tool that would allow agents to use computers to guess passcodes on an iPhone until they got one right.
Apple has warned that if the FBI gets what it wants, the software to disable an iPhone's security could get into the wrong hands and be used for nefarious purposes. At least one family directly affected by the shooting has said they support Apple's resistance.
The company last Thursday took a stand against the FBI when it filed a motion to vacate the California magistrate judge's order to help the government. Tuesday's formal objection was filed as an added legal layer to bolster Apple's case.
On Wednesday morning, it filed an additional notice in support of the motion to vacate.
FBI Director James Comey, however, defended his agency's request in a hearing Tuesday before the House Judiciary Committee.
In response to lawmaker questions, Comey said that the San Bernardino case was primarily about investigating the massacre, not establishing a legal precedent — a concern that's been raised by Apple and independent privacy advocates. Still, the director acknowledged that any ruling made by the California court would likely be weighed in any similar future cases.
His testimony and Apple's subsequent filing came a day after a federal judge in New York ruled that the company does not have to help investigators there unlock an Apple device in a drug case.
The U.S. District Court for the Central District of California will hear from both parties in the Apple vs. FBI case at a hearing on March 22.