Forcing Apple to help unlock an iPhone is a "modest" demand that may turn up vital evidence in a terrorist attack, the U.S. government argued Thursday, upping the ante in its legal standoff with the technology giant. A brief filed by the U.S. Justice Department was the latest shot in a battle that has fueled intense debate on whether Apple must comply with a court order to help the FBI break into a phone used by one of the shooters in the December terror attack in San Bernardino, California. The filing was a legal response to a challenge by Apple, which is backed by a broad coalition of powerful rival technology firms and activists and argues that the FBI is seeking a "back door" into all iPhones as part of the probe. The government brief, in sharp contrast, argued it is a single case of technical assistance in an important national security investigation."The court's order is modest. It applies to a single iPhone, and it allows Apple to decide the least burdensome means of complying," Justice Department lawyers wrote."It is a narrow, targeted order. ... The government and the community need to know what is on the terrorist's phone, and the government needs Apple's assistance to find out." An FBI victory in the case could serve as a legal precedent backing requests for access to iPhones by law enforcement agencies throughout the U.S. Apple general counsel Bruce Sewell slammed the brief as reading "like an indictment" and appearing crafted to smear the iPhone maker with innuendo such as implying a "sinister" relationship with China.