The FBI has a court order demanding Apple helps unlock an iPhone belonging to the gunman behind the San Bernardino terror attack, Syed Rizwan Farook.

Farook and his wife killed 14 people in the California city last December before police fatally shot them.

Family members of some victims have backed the FBI's order.

Two groups of tech giants have now filed an amicus brief, which allows parties not directly involved in a court case, but who feel they are affected by it, to give their view.

Apple has appealed against the court order, arguing that it should not be forced to weaken the security of its own products.