The U.S. government is warning that groups could rely on radio equipment as an alternative to social media to plan future criminal activities.

In a stark warning Sunday, the Federal Communications Commission's enforcement bureau said people coordinating or conducting criminal activity over radio waves are breaking the law.

"The Bureau has become aware of discussions on social media platforms suggesting that certain radio services regulated by the Commission may be an alternative to social media platforms for groups to communicate and coordinate future activities," the FCC said in its warning Sunday. "Individuals using radios in the Amateur or Personal Radio Services in this manner may be subject to severe penalties, including significant fines, seizure of the offending equipment, and, in some cases, criminal prosecution."

The FCC licenses certain signals for people to broadcast over radio waves. Those messages are generally protected by the U.S. Constitution's First Amendment. But the FCC reminded radio licensees and operators that it is prohibited to transmit "communications intended to facilitate a criminal act." People are also not allowed to encode their messages to obscure their meaning from law enforcement.

The laws governing airwaves apply to amateurs broadcasting with personal ham radios, which can reach long distances. But they also apply to people using Citizens Band (CB) radios commonly used for communication between truckers -- or even walkie-talkies.

In the wake of the January 6 Capitol riots, Facebook, Twitter and other mainstream social networks have become more vigilant about policing people who use their platforms to plan or incite attacks. They have booted off several high-profile radicals and thousands of groups and users who the platforms say engage in harmful conspiracy theories and other violence or hate speech.

Similarly, Amazon, Apple and Google effectively took Parler off the internet. Parler, the alternative social network popular with conservatives, had been surging in popularity in recent months. But the platform failed to rein in hate-filled, violent speech, Big Tech companies allege. Amazon, Apple and Google said that unmoderated speech could lead to another violent attack.

In response, Parler sued Amazon last week, alleging an antitrust violation, breach of contract and interference with the company's business relationships with users. The complaint calls Amazon Web Services' decision a "death blow" to Parler.

"Without AWS, Parler is finished as it has no way to get online," the complaint said. "And a delay of granting this TRO by even one day could also sound Parler's death knell as President Trump and others move on to other platforms."

Amazon said that Parler's lawsuit has "no merit."