Less than three weeks before Apple and the Justice Department bring their fight to a California courtroom, a wave of major technology firms and outside groups have linked arms and said they are united against the federal government’s demands.

Despite uncertainty among some in the tech industry over whether this was the fight to pick, a slew of big names had either signed on or announced plans to do so on Thursday, many of them worrying about what impact the case could have on privacy going forward.

Some relatives of some of the people killed in the Dec. 2 attack in San Bernardino, Calif., however, filed their own brief Thursday, siding with the FBI and arguing that accessing the phone may help answer questions about the attack.

“What if it leads to an unknown terrorist cell?” Mark M. Sandefur, whose son was killed in the attack, wrote in a letter to Apple chief executive Tim Cook that accompanied the filing. His letter alluded to the lingering belief some have that there was a third shooter in the attack. “What if others are attacked, and you and I did nothing to prevent it?”

Tech giants like Google, Facebook, Amazon.com, Yahoo and Microsoft are among those who signed a joint brief backing Apple. (Disclosure: Amazon founder Jeffrey P. Bezos owns The Washington Post.)

“Yahoo is proud to support Apple in this case,” Ron Bell, the company’s general counsel, said in a statement.

This brief — also signed by Dropbox, Evernote, Mozilla, Snapchat and WhatsApp, among others — said that the government’s requests would force companies to act “against their will” and change their products in a fundamental way.

“Asking a single company to undermine the security of its product for an investigation threatens the security of all of us in the long run,” Jan Koum, chief executive of WhatsApp, a messaging service owned by Facebook, said in a statement.

Twitter filed a joint brief on Thursday with eBay, Reddit and more than a dozen other tech companies arguing that the government’s request is “unbound by any legal limits” and “would set a dangerous precedent.”

The sheer level of support showed how pivotal the debate over an iPhone used by one of the San Bernardino attackers has become in Silicon Valley, as it has blossomed into a public fight over balancing privacy and security.

After FBI agents investigating the San Bernardino attack — which killed 14 people and injured 22 others — determined they could not access the locked phone, authorities sought and obtained a magistrate judge’s order demanding Apple’s help. The order directed Apple to write software disabling a feature that erases the phone’s data after 10 incorrect attempts at trying a password.

Apple has fought that order through public statements, court filings and, this week, a formal objection filed in court.

The Justice Department has said its demands in this case are specific, limited and could offer information for a terrorism investigation. Apple has argued that the case has much broader implications, both for its products and the tech industry as a whole.

On March 22, the federal government and Apple will continue this fight during oral arguments in the court case. But with that date looming, groups faced a deadline for filing amicus briefs adding their voices to the court record.

Law enforcement groups on Thursday gave their backing to the FBI, and while it was not the flood of support Apple saw from its tech industry brethren, the briefs came from several national as well as local groups.

The Federal Law Enforcement Officers Association, Association of Prosecuting Attorneys and National Sheriffs’ Association filed a joint brief saying that Apple’s stance has “real world, on-the-ground implications,” while the Justice Department’s arguments were also echoed by the California State Sheriffs’ Association, California Police Chiefs’ Association, California Peace Officers’ Association and the San Bernardino County District Attorney’s office.

A spokesperson for the Justice Department did not respond to a request for comment.

Some of the tech firms that ultimately decided to support Apple did so despite reservations by some of their executives, according to tech industry lawyers. These executives felt Apple had picked a legal battle that could backfire and hurt the industry, said several lawyers who have spoken to the executives but asked to remain anonymous to discus these talks.

The view was that this case was not the showdown over forcing a company to build a back door around strong, unbreakable encryption, one lawyer said. Still, these executives ultimately felt that “a bad ruling here will empower law enforcement to make all kinds of requests,” said one lawyer, who spoke on condition of anonymity to discuss confidential matters.

A lawyer for Apple said he has not heard any qualms from the firms he has dealt with who are filing amici. “All are 100 percent supportive,” he said. The only question he heard, he said, was whether Cook’s use of the word “backdoor” in his public statements was accurate.

In Apple’s view, the lawyer said, “what the government is seeking here is a backdoor to allow it to hack users’ systems….It’s weakening security. The encryption is protected by a passcode. So if you make it easier to guess the passcode, you are weakening the encryption.”

Al Gidari, a former partner at Perkins Coie who still speaks with tech executives, said there may have been initial discomfort about the case, given that it involved a terrorist attack and a “general sense” that the government’s request was not overly burdensome.

But Apple’s brief, in which it explained the burden and the harm, particularly in regard to the compelled signing of the software update, “has galvanized industry support,” said Gidari, who is now with Stanford’s Center for Internet and Society.

The American Civil Liberties Union said in its brief that the government is seeking authority “with catastrophic consequences for digital security and privacy.” The Consumer Technology Association, a trade group, joined other policy and advocacy groups in a brief that expressed worries about how other devices, manufacturers and companies would face similar demands if Apple lost this case.

In another brief, the Electronic Frontier Foundation — along with 46 technologists, security researchers and cryptographers — argued, as Apple has, that the judge’s order would violate the company’s First Amendment rights.

Dropbox said it would file a brief supporting Apple because it was taking a stand against attempts to “undermine the security of a company’s products,” Ramsey Homsany, the company’s general counsel, said in a statement.

AT&T filed its own amicus brief supporting Apple because it believes Congress must pass new legislation to provide clarity about what authorities can seek, David McAtee, the company’s senior executive vice president and general counsel, said in a statement Thursday.

One noticeable absence from the companies backing Apple: Verizon, which industry lawyers had said earlier this week would be among those filing briefs.

Lowell McAdam, chief executive of Verizon, wrote in a column posted on LinkedIn that while he was not taking a side in the Apple case, the company believed in “strong encryption with no ‘back doors’ that would enable government access to private information.”

McAdam said he believed Congress should step in to define the specific cases where data must be preserved for investigations, rather than letting this be determined “haphazardly” or by “one judge in one region of the U.S.” A spokesman did not respond to a request for further comment.

An attorney representing the families of five people killed in the San Bernardino attack and another person who survived the attack filed a brief arguing that Apple is “grandstanding” and should be required to comply with the court order.

“This case is not what Apple is making it out to be,” the filing states. “To obtain sympathy for its cause, Apple would like to portray this case as one in which the privacy interests of millions of Americans are at stake.”

Despite Apple’s claims, “that is not what this case is about” because the government’s request is so specific and limited, the filing states.

In another letter, addressed to the magistrate judge and posted on Apple’s website, a man whose wife was slain in the San Bernardino attack said he supported the company. He wrote that he shared the company’s fears that “this software the government wants [Apple] to use will be used against millions of other innocent people.”

Ellen Nakashima contributed to this report.

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This story has been updated.