Apple CEO Tim Cook says the company will fight an order that instructs it to provide “reasonable technical assistance” to unlock an iPhone that belonged to one of the San Bernardino attackers.

In a letter to Apple’s customers, Cook writes that opposing the “order is not something we take lightly … [but] we feel we must speak up in the face of what we see as an overreach by the U.S. government.”

At issue is a court order issued Tuesday by Magistrate Judge Sheri Pym of the Federal District Court for the District of Central California ordering Apple to, in the words of The Associated Press, “supply highly specialized software the FBI can load onto the phone to cripple a security encryption feature that erases data after too many unsuccessful unlocking attempts.” Wired adds that Apple’s compliance would allow the FBI to attempt to unlock the phone using multiple password attemptsa method known as bruteforcing. But Apple declined, calling for a public discussion, so its customers and citizens “understand what is at stake.”

The phone in question is the iPhone 5c that belonged to Syed Rizwan Farook, who with his wife, Tashfeen Malik, opened fire at a civic-services center in San Bernardino,  California, killing 14 people and wounded 21 others. The attack was believed to be inspired by ISIS, though no direct link has been found between the attackers and the Sunni terrorist group.

Cook, in his letter, said the company has cooperated with the FBI’s investigation into the attack, complied with subpoenas and search warrants in the case, and made Apple engineers available to advise the bureau.

“But now the U.S. government has asked us for something we simply do not have, and something we consider too dangerous to create,” he said. “They have asked us to build a backdoor to the iPhone.”

Specifically, the FBI wants us to make a new version of the iPhone operating system, circumventing several important security features, and install it on an iPhone recovered during the investigation. In the wrong hands, this software — which does not exist today — would have the potential to unlock any iPhone in someone’s physical possession.

The FBI may use different words to describe this tool, but make no mistake: Building a version of iOS that bypasses security in this way would undeniably create a backdoor. And while the government may argue that its use would be limited to this case, there is no way to guarantee such control.

Apple points out that the FBI—rather than seeking congressional legislation—is seeking a new interpretation of the All Writs Act of 1789, which allows judges to “issue all writs necessary or appropriate in aid of their respective jurisdictions and agreeable to the usages and principles of law.”

Cook say the government could use order this to demand that the company build surveillance software to intercept customers’ messages, access their health records or financial data, track their location, or even access a phone’s microphone or camera without its owner’s knowledge.

“The implications of the government’s demands are chilling,” Cook writes. “If the government can use the All Writs Act to make it easier to unlock your iPhone, it would have the power to reach into anyone’s device to capture their data.”