Getty Images/Photononstop RM

Thieves, consider yourselves on notice: California is now smartphone "kill switch" territory.

The so-called software is designed to make stealing smartphones essentially pointless by allowing owners to remotely lock their device so no one can use it. The technology, which includes Apple's "Activation Lock" and Google's "Device Protection," has become a key selling point among phone manufacturers that offer peace of mind to protect customers' information if a phone is stolen, and hopefully discourage thieves from stealing it in the first place.

There's good reason for these features. In the past several years, government officials have noticed an "epidemic" of phone thefts, particularly in large cities. Thieves often steal phones and sell them to cartels and shops that often shipped them to willing customers overseas.

The technology industry's answer has been to create software that responds to a theft by requiring users to input a passcode before it can be unlocked or restored to factory settings. The technology looks to be working: In 2013, 3.1 million Americans had their phones stolen, according to a study published by Consumer Reports last month. Last year, that number fell to 2.1 million, according to the report.

Aside from the dropping theft numbers, there's another indication the kill switch is working. Last year, CNET interviewed a former smartphone thief about his motivation for stealing phones. He targeted the devices because it was "a lot faster" than other crimes. But kids might think twice about stealing phones if they knew they couldn't be unlocked, he said.

James Martin/CNET

The next step was making sure people used the technology. California Gov. Jerry Brown signed a bill last August mandating kill switch software be included and turned on in all smartphones manufactured after Wednesday and sold in the state.

The bill, which was sponsored by California state Sen. Mark Leno and San Francisco District Attorney George Gascon, received some pushback from the wireless industry before it was passed. The industry argued a kill switch could be exploited by hackers, but supporters of the law have questioned the motivations of mobile carriers, which have lucrative deals with insurance partners.

"For years consumers have been victimized because the wireless industry failed to safeguard its products," Gascon said in an email. "Today that changes."

On Wednesday, the CTIA, a trade organization that represents the wireless industry, cheered on the drop in theft. Meredith Attwell Baker, the group's CEO and president, said in a statement that adding theft-deterrent software to phones is "another example of the wireless industry proactively working together with policymakers and law enforcement to help protect consumers' smartphones."

California is the second state to mandate kill switch software. Last May, Minnesota was first to pass a kill switch bill, but that law does not require the feature come turned on as the default setting when a consumer initially sets up his or her new smartphone -- a distinction supporters of the California law deem very important.

Now it is up to retailers to to comply with the law. The price tag for the "knowing retail sale" of a smartphone that doesn't meet the requirement is $500 to $2,500, according to the bill. Walmart -- one of the biggest retailers in the US -- said it would comply with the law. Best Buy, another major US retailer, didn't respond to a request for comment about how ready its California stores are to comply.

Though the rule went into effect today, government officials say it will take some time for it to have the maximum effect. One of the problems with Google's Android software is that many phones don't run the most current versions of the software, often because manufacturers are slow to push out updates, a challenge observers refer to as "fragmentation."

"As this technology is implemented ubiquitously, and as older phones are slowly phased out, I expect this epidemic to become a thing of the past," said Gascon.