More: Some wear balaclavas. Some wear neck gators, sunglasses and hats. Some wear masks and casual clothes.

Across the country, armed federal immigration officers have increasingly hidden their identities while carrying out immigration raids, arresting protesters and roughing up prominent Democratic critics.

It's a trend that has sparked alarm among civil rights and law enforcement experts alike.

Mike German, a former FBI agent, said officers' widespread use of masks was unprecedented in US law enforcement and a sign of a rapidly eroding democracy. "Masking symbolizes the drift of law enforcement away from democratic controls," he said.

The Department of Homeland Security (DHS) has insisted masks are necessary to protect officers' privacy, arguing, without providing evidence, that there has been an uptick in violence against agents.

But, German argued, the longterm consequences could be severe. The practice could erode trust in the US law enforcement agencies: "When it's hard to tell who a masked individual is working for, it's hard to accept that that is a legitimate use of authority," he noted.