Top technology companies Google, Microsoft, Facebook and AT&T are filing legal briefs supporting Apple in a high-profile fight against the U.S. Justice Department's bid to unlock an encrypted iPhone belonging to one of the San Bernardino shooters.

The dispute between Apple and the government intensified last month when the Federal Bureau of Investigation obtained a court order requiring Apple to write new software and take other measures to disable passcode protection and allow access to shooter Rizwan Farook's iPhone.

Apple has argued that such a move would set a dangerous precedent. 

AT&T filed its brief Thursday.

Internet heavyweights to support for Apple

Alphabet's Google, Facebook, Microsoft and about a dozen other internet companies will file a joint legal brief on Thursday asking a judge to support Apple in its battle with the U.S. government, sources familiar with the companies' plans said.

The effort is a rare display of unity and support for the iPhone maker from companies that are competitors in many areas, and shows the breadth of Silicon Valley's opposition to the government's anti-encryption effort.

The group plans to file what is known as an amicus brief — a form of comment from outside groups common in complex cases — to Riverside, Calif., federal judge Sheri Pym, who will rule on Apple's appeal of the court order. 

Mozilla, maker of the Firefox web browser, said it was joining in the effort along with online planning tool maker Evernote and messaging app firms Snapchat and WhatsApp. Photo sharing service Pinterest and online storage firm Dropbox are also participating.

"We stand against the use of broad authorities to undermine the security of a company's products," Dropbox general counsel Ramsey Homsany said in a statement.

Networking leader Cisco Systems said it expected to address the court on Apple's behalf, but did not say whether it was joining with the large group of companies.

Semiconductor maker Intel plans to file a brief of its own in support of Apple, said Chris Young, senior vice-president and general manager for Intel Security Group.

"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," Young said in an interview.

The Stanford Law School for Internet and Society filed a separate brief on Thursday morning 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.

Syed Rizwan Farook and Tashfeen Malik

U.S.-born Syed Rizwan Farook, 28, and his spouse, Tashfeen Malik, 29, a native of Pakistan who lived in Saudi Arabia for more than 20 years, died in a shootout with police hours after an attack on a holiday party at the Inland Regional Center social services agency in San Bernardino, Calif. (FBI/Reuters)

"The dangers of forcing companies to denigrate the security of their products and of allowing law enforcement to commandeer consumer devices for surveillance purposes are too great," the brief said.

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.

Relatives of 6 victims side with government

Briefs are also expected in support of the government. Stephen Larson, a former federal judge, told Reuters last week that he is working on a brief with victims of the San Bernardino shooting who want the U.S. Federal Bureau of Investigation to be able to access the data on the phone used by Rizwan Farook, one of the shooters.

"They were targeted by terrorists, and they need to know why, how this could happen," Larson said.

Six relatives of San Bernardino attack victims filed a legal brief on Thursday opposing Apple in its high-profile fight.

In the families' brief, they argue that Apple's arguments are misplaced because the government has a valid warrant, and "one does not enjoy the privacy to commit a crime."

The families also said Apple "routinely modifies its systems" to comply with Chinese government directives.

But not all the victims support the government. Salihin Kondoker, whose wife Anies Kondoker was injured in the 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.

"I believe privacy is important and Apple should stay firm in their decision," the letter said. "Neither I, nor my wife, want to raise our children in a world where privacy is the tradeoff for security."

The clash has intensified a long-running debate over how much law enforcement and intelligence officials should be able to monitor digital communications.

Law enforcement officials have said that Farook and his wife, Tashfeen Malik, were inspired by Islamist militants when they shot and killed 14 people and wounded 22 others on Dec. 2 at a holiday party. Farook and Malik were later killed in a shootout with police.  

The FBI said it wants to read the data on Farook's phone to investigate any links with militant groups.