It took two days, but the companies' entry solidifies a small but powerful band of tech giants supporting Apple in its quest to buck government demands that it says would irreparably damage security and erode consumer trust.
In a statement, Facebook said, "We will continue to fight aggressively against requirements for companies to weaken the security of their systems."
The case has intensified the rift between tech companies and law enforcement over the limits of encryption. And law enforcement groups have been vocal about their support for the Justice Department.
Although some firms have remained silent, the industry is firmly on Apple's side, said Aaron Levie, CEO of cloud-based storage provider Box Inc.
"Companies choose to use their political capital when it is really important or relevant to them," Levie said. "If individually pressed you would see the same message from essentially any Internet or hardware or enterprise software CEO or company, and that's because the fundamental security model of our technology would break if you were to comply with this kind of order."
Levie said he unequivocally supports Cook's stance.
"The whole grounds on which Apple is standing on are just super important," he said.
At the center of the case is an iPhone used by Rizwan Farook, who along with his wife, Tashfeen Malik, killed 14 people and wounded 22 in a shooting rampage in San Bernardino, California.
The young married couple sympathized with Islamic State militants, and government investigators want the data on the phone to learn more about their activities the day of the shooting and their contacts with either accomplices or Islamic State.
Apple's Cook had said the court's demand threatened the security of Apple's customers and had "implications far beyond the legal case at hand."
© Thomson Reuters 2016