Tech giants including Google, Facebook and Microsoft have backed Apple in its battle with US authorities over encryption on its devices
The FBI has a court order asking Apple to build a software to unblock the smartphone used by one of the perpetrators of a December shooting in San Bernardino, California.
In a rare show of unity from Silicon Valley, Google, Facebook and Microsoft, plus more than two dozen other internet and tech firms, have filed legal papers asking a judge to support Apple's position.
Apple has argued that disabling the password protection and allowing access to the device would set a dangerous precedent and threaten customer security.
It has also claimed that it is a matter of free speech in that computer code is a form of expression.
It follows a long-running debate over how much governments and law enforcement should be able to monitor digital communications.
Family members of some of the victims of the San Bernardino shooting, which claimed 14 lives, have sided with the government.
They say Apple's arguments were misplaced because the government had a valid warrant, and "one does not enjoy the privacy to commit a crime."
The families have also asserted that Apple "routinely modifies its systems" to comply with Chinese government directives.
The San Bernardino shootings were carried out by Rizwan Farook and his wife, Tashfeen Malik, who were inspired by ISlamist militants.
They couple were killed following the attack in a shootout with police.
The FBI wants access to their phone data to examine any links with militant groups.
Additional reporting by Reuters
