Four out of five iPhone users have enabled a feature that can locate, lock or wipe a lost or stolen device, the San Francisco district attorney's office says. The office conducted a survey in November as part of District Attorney George Gascon's effort to get smartphone makers to address the rising problem of smartphone theft in major U.S. cities, ITWorld reported Wednesday. In responses to a request for survey participants on social media, 245 out of 313 respondents said they had enables the activation-lock feature of Apple's Find my iPhone app. Find my iPhone allows phone owners see the location of their phone on a map, lock the phone and remotely erase personal data. Starting with the iOS 7 version of Apple's operating system, the software includes an activation lock which keeps the phone from being unlocked wiped or erased by a third party without the use of a password. Gascon has said he believes if smartphones can be rendered useless after they are stolen, their resale value disappears and so does the incentive for theft. While he lauded Apple for providing the feature, he said more iPhone owners need to take advantage of it. "Until Activation Lock is fully opt-out, it appears many iPhone owners will not have the solution enabled," he said in a statement. "This leaves iPhone users at risk, as thieves cannot distinguish between those devices that have the feature enabled and those that do not."