The rare display of unity and support from Apple's sometime-rivals showed the breadth of Silicon Valley's opposition to the government's anti-encryption effort.
Apple's battle became public last month when the US Federal Bureau of Investigation obtained a court order requiring the company to write new software and take other measures to disable passcode protection and allow access to an iPhone used by one of the shooters in the December killings in San Bernardino.
Apple pushed back, arguing that such a move would set a dangerous precedent and threaten customer security. The clash has intensified a long-running debate over how much law enforcement and intelligence officials should be able to monitor digital communications.
Apple's industry allies, along with several privacy advocates, were filing amicus briefs - a form of comment from outside groups common in complex cases - to Riverside, California federal Judge Sheri Pym who had set a Thursday deadline.
Six relatives of San Bernardino attack victims on Thursday weighed in with their own Amicus brief opposing Apple.
The companies backing Apple largely echo the iPhone maker's main argument, that the 1789 All Writs Act at the heart of the government's case cannot be used to force companies to create new technology.
One amicus filing, from a group of 17 Internet companies including Twitter and LinkedIn, asserted that Congress had already passed laws that establish what companies could be obliged to do for the government, and that the court case amounted to an "end run" around those laws.
Apple, and some of the other briefs, did not go quite that far, but also asserted that Congress, not the courts, needed to address the issue. Congress has struggled without success for years to address law-enforcement concerns about encryption.
The victims' families argued that 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 also asserted that Apple "routinely modifies its systems" to comply with Chinese government directives.
Apple has also advanced a free speech argument, on the grounds that computer code is a form of expression and cannot be coerced. The families pushed back against that defence: "This is the electronic equivalent of unlocking a door - no expression is involved at all," they said.
Two big coalitions
The tech and Internet industries largely coalesced around two filings. One includes market leaders such as Google, Microsoft and Facebook, and smaller, younger companies such as Mozilla, Snapchat and Dropbox.
That group noted that Congress passed the All Writs Act before the invention of the light bulb, and that it goes too far to contend that the law can be used to force engineers to disable security protections.
The brief also highlighted a unanimous 2014 US Supreme Court case which said law enforcement needed warrants to access smartphones snared in an arrest, said a source familiar with the brief who declined to be named because it had yet to be filed.
That opinion, penned by Chief Justice John Roberts, united the Supreme Court's liberal and conservative factions.
The second industry coalition, which includes Twitter Inc, eBay Inc and LinkedIn Corp, contended in its filing that the Communications Assistance for Law Enforcement Act (CALEA) of 1994, along with other statutes, had already made it clear what the companies could or could not be forced to do.
CALEA requires telephone companies to allow interception of communications, but notably excludes "information service" companies from such mandates. Apple said it was rightly considered an information company in this context.
AT&T's filing, by contrast, called for a "new legislation solution" that "applies equally to all holders of personal information," an apparent reference to the exemption for information providers in CALEA.
Semiconductor maker Intel Corp also filed a brief of its own in support of Apple.
"We believe that tech companies need to have the ability to build and design their products as needed, and that means that we can't have the government mandating how we build and design our products," said Chris Young, senior vice president and general manager for Intel Security Group, in an interview.
The Stanford Law School Center for Internet and Society filed a separate brief on Thursday on behalf of a group of well-known experts on iPhone security and encryption, including Charlie Miller, Dino Dai Zovi, Bruce Schneier and Jonathan Zdziarski.
Privacy advocacy groups the American Civil Liberties Union, Access Now and the Wickr Foundation filed briefs on Wednesday in support of Apple before Thursday's deadline set by Pym.
Salihin Kondoker, whose wife Anies Kondoker was injured in the San Bernardino attack, also wrote on Apple's behalf, saying he shared the company's fear that the software the government wants Apple to create to unlock the phone could be used to break into millions of other phones.
Law enforcement officials have said that Rizwan Farook and his wife, Tashfeen Malik, were inspired by Islamist militants when they shot and killed 14 people and wounded 22 others last December 2 at a holiday party. Farook and Malik were later killed in a shootout with police and the FBI said it wants to read the data on Farook's work phone to investigate any links with militant groups.
Earlier this week, a Brooklyn judge ruled that the government had overstepped its authority by seeking similar assistance from Apple in a drug case.
© Thomson Reuters 2016