REUTERS/Amir Cohen WASHINGTON/SAN FRANCISCO, Dec 3 (Reuters) - Apple Inc iPhones of at least nine U.S. State Department employees were hacked by an unknown assailant using sophisticated spyware developed by the Israel-based NSO Group, according to four people familiar with the matter. NSO has long said it only sells its products to government law enforcement and intelligence clients, helping them to monitor security threats, and is not directly involved in surveillance operations. NSO software is capable of not only capturing encrypted messages, photos and other sensitive information from infected phones, but also turning them into recording devices to monitor surroundings, based on product manuals reviewed by Reuters. Since at least February, this software flaw allowed some NSO customers to take control of iPhones simply by sending invisible yet tainted iMessage requests to the device, researchers who investigated the espionage campaign said. Sen. Ron Wyden, who is on the Senate Intelligence Committee, said: "Companies that enable their customers to hack U.S. government employees are a threat to Americas national security and should be treated as such."