DOJ Escalates Battle With Apple Over Gunman's iPhone

The Justice Department is pushing forward with its legal fight against Apple, urging a federal judge to compel the tech giant to help the FBI crack open a cellphone left behind by one of the San Bernardino, California, shooters.

"Rather than assist the effort to fully investigate a deadly terrorist attack by obeying this court's [previous order], Apple has responded by publicly repudiating that order," prosecutors wrote in a new filing today.

Syed Farook, who along with his wife, Tashfeen Malik, launched a deadly assault on Dec. 2, 2015, killing 14 of Farook's coworkers at a holiday party.

At the Justice Department's request earlier this week, U.S. Magistrate Judge Sheri Pym in California ordered Apple to help the FBI crack open Farook's iPhone.

Federal prosecutors say the phone, given to Farook by his employer, could be hiding "crucial evidence" about the terror attacks.

"The government requires Apple's assistance to access the ... device to determine, among other things, who Farook and Malik may have communicated with to plan and carry out the IRC shootings, where Farook and Malik may have traveled to and from before and after the incident, and other pertinent information that would provide more information about their and others' involvement in the deadly shooting," prosecutors said in their initial filing on Tuesday.

Prosecutors said Farook's device could be encrypted to the point that its content would be "permanently inaccessible," and, "Apple has the exclusive technical means which would assist the government in completing its search."

"Apple's current refusal to comply with the Court's Order, despite the technical feasibility of doing so, instead appears to be based on its concern for its business model and public brand marketing strategy," the government filing said.

After Pym issued her order late Tuesday demanding that Apple assist federal agents, Apple quickly vowed to challenge the decision.

"The United States government has demanded that Apple take an unprecedented step which threatens the security of our customers," Apple CEO Tim Cook said in a statement to customers. "[T]his order ... has implications far beyond the legal case at hand."

"The FBI wants us to make a new version of the iPhone operating system, circumventing several important security features, and install it on [the shooter's] iPhone," Cook added. "In the wrong hands, this software - which does not exist today - would have the potential to unlock any iPhone in someone's physical possession."

In addition, all of the personal and sensitive information on customers' phones "needs to be protected from hackers and criminals who want to access it, steal it, and use it without our knowledge or permission," Cook wrote.

If the battle between the FBI and Apple continues, it's a matter that could work its way up to the U.S. Supreme Court.

ABC News' Kelly Stevenson and Julia Jacobo contributed to this report.