Searching on Google's shopping site showed dozens of links to devices that are advertised as signal jammers, including anti-drone devices. Google itself does not sell them, but instead directs shoppers to third-party sellers, such as electronics retailers in China. These retailers typically say they can ship to the United States, and their marketing materials are not specifically for military or law enforcement use. One Chinese seller posted a video on YouTube, which is owned by Google, demonstrating how its product blocks cell phone signals.

A Google spokesperson said the company has reviewed the cases shared by NBC News and is removing listings that violate its policy prohibiting the sale or promotion of such devices.

Sometimes it's unclear who is behind the online sellers. One retailer called itself "Professional Global Signal Jammer Manufacturer Online Jammer Store" but did not disclose its location. The company did not respond to a request for comment. The FAQ section of its website includes the following dialogue:

"Will customs confiscate my package?"

"So far, we have not encountered this situation."

It is unclear how many people may have illegally purchased radio frequency drone jammers from the various websites reviewed by NBC News. At least some of the companies said they sell their products to customers outside the United States, where the FCC's strict rules don't apply.

At the other end of the radio signal jammer market are military suppliers, government contractors and other companies that don't generally target their products at consumers. Many of these companies have websites marketing their products, but they don't list prices or offer options to buy them immediately. Instead, they list options to "request a consultation" or "contact an expert."

An FCC spokesman pointed to the commission's standing advisory on law enforcement and a 2020 federal interagency advisory on drone defense.

On Wednesday, after NBC News published the report, the FCC said it had received several investigations related to jammer marketing, including one involving Amazon.

"We are conducting several investigations into retailers, including Amazon, for possible violations of the commission's rules by marketing and selling devices without proper FCC authorization," FCC spokesman Jonathan Uriarte said in a statement.

Amazon did not immediately respond to a request for comment on the investigations on Wednesday.

The FCC has cracked down on radio frequency jammers and their online sales. In 2016, the commission said it had fined a Chinese online retailer, CTS Technology, $34.9 million for selling hundreds of illegal jammers over two years and selling some to undercover FCC agents.

There have been no similar fines in the U.S. since then, though other Chinese retailers still market radio jammers to Americans online, an NBC News review of their websites shows. NBC News could not determine if any of them have shipped products to the U.S.

The FCC has also targeted buyers in past years. In 2018, the FCC said it fined the operator of a Dallas warehouse $22,000 for using a jammer to prevent employees from using their cell phones while working. In 2016, the commission said it fined a Florida man $48,000 for using a jamming device to prevent people in other vehicles from talking on their cell phones during his daily commute. In 2014, Marriott agreed to pay $600,000 to settle allegations that one of its properties in Nashville, Tennessee, interfered with Wi-Fi signals at non-Marriott hotels.

Over the years, the FCC’s focus has shifted to other topics, such as combating robocalls, said Dale Hatfield, a former chief engineer at the commission.

“Their enforcement activity seems to be more targeted at things that are more directly visible to consumers,” he said.

It’s hard to determine how big a problem illegal jammers are because signal interference is often brief and hard to document, said Hatfield, an adjunct professor at the University of Colorado Boulder. For the same reason, he said, interference cases are difficult for the FCC to handle.

“It can be very intermittent,” he said. “How do you go back and figure out why?”

Former FCC Commissioner McDowell said the commission’s enforcement efforts are largely determined by the types of complaints it receives.

“Typically, the enforcement process comes into play when an outside agency notifies the FCC of a situation,” he said.

Jamming devices have many potential uses, from disrupting unwanted drones to disrupting Wi-Fi networks to disabling doorbell cameras. On a Reddit message board dedicated to Ring doorbell cameras, some posters complained about people using jammers to disrupt the cameras and avoid detection during thefts. One Reddit poster said the irony was that Amazon seemed to be profiting from both sides: Ring is a division of Amazon, and Amazon is also a marketplace for buying devices that can disable Ring cameras.

Amazon did not respond to questions about jamming Ring devices.

Jammers, or radio frequency "blocking," are also a frequent topic of discussion in online forums dedicated to people who believe that the government or others are conducting deep surveillance on them, a belief or phenomenon sometimes referred to as "gang stalking."

Potential markets for anti-drone systems such as RF jammers include large businesses and organizations that are concerned about drone terrorism but are not legally allowed to use jammers. Last year, four major sports leagues, including the NFL, urged Congress in a letter to congressional leaders to allow state and local law enforcement to use anti-drone capabilities.

Prison officials in some states have said they want legal authority to use jammers to prevent prisoners from using their phones.

But under current law, the list of government agencies authorized to buy and use radio frequency jammers is short, including the FBI and the Department of Homeland Security, as well as the Departments of Energy and Defense.

Companies that specialize in drone countermeasures often focus on foreign buyers.

Drone defense company IXI Electronic Warfare warns on its website that drones pose a potential threat to stadiums, prisons and airports, and says drones can help "eliminate threats in seconds," including using a radio frequency jammer called a "Dronekiller." But overall, stadium operators, prisons and airport authorities do not have the legal authority to buy such products in the U.S.