(Telecompaper) Apple has rejected a request from the US government to build a 'backdoor' to the iPhone operating system in order to aid a terrorism investigation. The case concerns the attack in San Bernardino, California in December 2015. Apple CEO Tim Cook said in a post on the company's website that Apple had already cooperated with the FBI on the case, providing requested data, complying with warrants and subpoenas and making its engineers available to help investigate mobile devices possibly used by the suspects. However, the company has drawn the line at a request from the FBI to make a new version of the iPhone operating system that would circumvent several important security features, Cook said. This would be installed on an iPhone found during the investigation in an attempt to recover further information. Cook said such a 'backdoor' does not exist today, and the company is concerned that if created, such software could fall into the wrong hands. The existence of such a passcode to the iOS system would expose all Apple's customers to potential security risks and privacy invasion.