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.

Relatives of some of the people killed in the Dec. 2 attack in San Bernardino, California, 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. “What if others are attacked, and you and I did nothing to prevent it?”

Tech giants such as Google, Facebook and Microsoft are among those signing onto court briefs backing Apple, according to Mozilla, which said it was joining those firms in a friend-of-the-court brief. 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.”

Yahoo planned to file an amicus brief Thursday, and the company “is proud to support Apple in this case,” Ron Bell, the company’s general counsel, said in a statement.

Industry lawyers said that Amazon.com also intended to file or join a brief, but a spokesman for the company did not immediately respond to a request for comment. (Amazon founder Jeffrey P. Bezos owns The Washington Post).

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, both sides will continue the 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.

Some of the firms that ultimately decided to file briefs did so despite reservations by some of their executives, according to tech industry lawyers. These executives thought that 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 discuss these talks.

Still, these executives ultimately thought that “a bad ruling here will empower law enforcement to make all kinds of requests,” said one lawyer, who spoke on the 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 that are filing the briefs. “All are 100 percent supportive,” he said. The only question he heard, he said, was whether Cook’s use of the term “back door” in his public statements was accurate.

In Apple’s view, the lawyer said, “what the government is seeking here is a back door 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.”

A lawyer 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.

(c) 2016, The Washington Post · Mark Berman