The ACLU said that it supports Apple in its court battle against the FBI.
In a brief filed to the federal court handling Apple's case, the ACLU said that forcing Apple to write software is akin to making Apple become part of the FBI itself.
"Law enforcement may not commandeer innocent third parties into becoming its undercover agents, its spies, or its hackers," the ACLU wrote in its brief.
Apple is refusing to comply with a court order to create a software program that will help the FBI unlock an iPhone used by San Bernardino shooter Syed Farook.
Citing the First Amendment, Apple claims it cannot be compelled to write software. Apple says the software would amount to a backdoor that could potentially be used to unlock millions more iPhones.
In its filing, the ACLU agreed.
"If the government prevails, then this case will be the first of many requiring companies to degrade the security and to undermine the trust in their products," the ACLU said. "For the many users who rely on digital devices to secure their information ... this burden would be severe."
Anthony Romero, executive director of the ACLU, told CNNMoney that the implications of Apple losing its court battle would be vast. He said dissidents would no longer feel safe communicating on their iPhones, because governments could spy on their communications by exploiting a backdoor that Apple would be forced to create.
He said he is pleased that Apple is "drawing a line in the sand."
Amazon (AMZN), Facebook (FB), Google (GOOGL), Microsoft (MSFT) and other tech companies will also file documents with stating that they stand with Apple, according to the companies and sources familiar with their plans.
The briefs are due to the court no later than Thursday.
