Apple CEO Tim Cook has posted “A Message to Our Customers” (http://www.apple.com/customer-letter/) saying that Apple opposes an order from the U.S. government to unlock a shooter’s iPhone.

U.S. Magistrate Judge Sheri Pym has ordered Apple to help the FBI break into an iPhone belonging to Syed Farook, one of the shooters in the Dec. 2 attack that killed 14 people. The ruling by Pym requires Apple to supply software the FBI can load onto Farook’s county-owned work iPhone to bypass a self-destruct feature that erases the phone’s data after too many unsuccessful attempts to unlock it. The FBI wants to be able to try different combinations in rapid sequence until it finds the right one.

The Apple CEO says that “Building a version of iOS that bypasses security in this way would undeniably create a back door” and “while the government may argue that its use would be limited to this case, there is no way to guarantee such control.”