In a major step toward curbing smartphone theft, Google and Microsoft will incorporate kill switches into the next versions of their operating systems for mobile devices, enabling owners to render the tech products useless if stolen, San Francisco District Attorney George Gascón said Thursday.

Google's operating system, Android, runs on more than half of all smartphones used in the United States. Microsoft's operating system is on all Nokia smartphones. Apple incorporated its version of a kill switch last year for its iPhones and other mobile devices.

Combined, the three systems cover 97 percent of the smartphones in the United States. The companies' decision will make the devices far less attractive to thieves, law enforcement officials said.

"The commitments of Google and Microsoft are giant steps toward consumer safety," New York Attorney General Eric Schneiderman said in a joint statement with Gascón announcing the changes.

The two prosecutors have been at the forefront of an international effort to combat cell phone theft by persuading manufacturers and wireless carriers to incorporate technology that can deactivate the devices if they are stolen.

What kill switches do

Kill switches let users remotely erase their personal data and render phones inoperable. If a phone is recovered, the owner can return it to working condition and restore the wiped data.

After Apple introduced its kill switch in September, robberies and grand larcenies involving iPhones plummeted, according to a report released Thursday.

Robberies of iPhones in San Francisco declined 38 percent in the six months after Apple rolled out its activation lock feature, according to Police Department figures cited in the report from a group that Gascón and Schneiderman formed last year.

Perhaps most tellingly, according to the prosecutors, robberies and grand larcenies involving Samsung smartphones, which did not have a kill switch during much of that time, increased in the city by 12 percent.

Same story in N.Y.

Similar patterns were seen in New York, where robberies and grand larcenies involving Apple products dropped 19 percent and 29 percent, respectively, in the first five months of 2014 compared with the year before, according to the report by a partnership of law enforcement and elected officials and consumer advocates.

The decrease was substantially greater than the overall decline in robberies and grand larcenies in New York - 10 percent and 18 percent, respectively. Robberies and grand larcenies involving Samsung smartphones increased by more than 40 percent.

'Effective deterrent'

Samsung introduced a kill switch in April for devices on the Verizon Wireless and U.S. Cellular networks.

"We can make the violent epidemic of smartphone theft a thing of the past, and these numbers prove that," Gascón said in a statement. "It was evident from day one that a technological solution was not only possible, but that it would serve as an effective deterrent to this growing threat."

Mobile devices are a prime target for robbers because of their high resale value on secondary markets. About 3.1 million people nationwide were victims of mobile device theft last year, according to Consumer Reports magazine.

In San Francisco, thefts of smartphones and other mobile devices accounted for two-thirds of robberies in 2013, Gascón said.

"If there were no cell phone robberies, violent crime would be down by double figures," Police Chief Greg Suhr said in January.

Leno's legislation

In May, the state Senate approved legislation authored by Sen. Mark Leno, D-San Francisco, that would require all smartphones sold in California to be equipped with theft-deterrent technology that is engaged by default, a mandate that the wireless industry's trade group says is unnecessary.

Leno's bill would require that the kill switch be an opt-out feature - that is, consumers would have to turn it off to disable it, rather than taking steps to activate it, or opt in.

Specifics of the deterrent options being rolled out by Microsoft and Google were not immediately available, but one source involved in the process said Microsoft was expected to make its kill switch an opt-out feature.

Stands firm on opt-out

Leno, whose bill has passed the Senate and is now before an Assembly committee, said the move by Google and Microsoft "vindicates and validates those of us who have been working on this legislatively all year."

"We are insistent, and will remain so, that our bill will require an opt-out provision, because it's fundamental," Leno said. "We want the criminal to understand there is no value for his risk of assaulting someone for their phone. ... That will finally deter this whole shift in criminal behavior."

CTIA-the Wireless Association, a trade group for wireless providers, had said kill switches leave devices open to hacking and raise privacy concerns. In April, the group said providers would implement a kill switch on an opt-in basis.

Fighting Leno's bill

Jamie Hastings, CTIA's vice president for external and state affairs, described Google's and Microsoft's announcements as "the latest example" of the wireless industry's dedication "to consumer security and to help law enforcement with its stolen phone issue."

Leno said CTIA is still fighting his bill.

Gascón contends wireless carriers' opposition is rooted in money.

The top four U.S. wireless carriers make about $7.8 billion in annual revenue from insurance protection plans, and kill switches would cost the industry an estimated $2.5 billion on insurance products alone, Gascón said.