WATCH: US Marines Carry Out First-Known Detainment of Civilian

In an unprecedented move, U.S. Marines temporarily detained a civilian in Los Angeles, the first confirmed instance of active-duty troops carrying out such an action on domestic soil during Donald Trump's recent military deployment.

Tensions have escalated in Los Angeles following days of public outcry and protest against immigration raids led by ICE agents. In response, President Donald Trump authorized the deployment of active-duty Marines and National Guard troops to protect federal properties and personnel, including the Wilshire Federal Building.

The rare domestic use of active-duty forces raised alarm about the military's expanding role in civilian affairs, especially amid growing speculation that the White House might invoke the Insurrection Act.

On Friday, Marines assumed control of the Wilshire Federal Building's security. Shortly after, Reuters captured images of several Marines detaining a man later identified as Marcos Leao, 27, who said he was visiting the Department of Veterans Affairs nearby.

Leao, an Army veteran and naturalized citizen, stated he crossed a yellow-tape barrier without realizing the restrictions. He was restrained with zip ties and later handed off to Homeland Security officers. Leao told reporters after his release that the troops "treated [him] very fairly" and that he believed they were simply doing their job.

U.S. Northern Command confirmed the incident and clarified that active-duty troops may conduct temporary detentions if an individual presents a potential threat to federal property or personnel, but such custody must end as soon as civilian law enforcement can intervene.

Marines and National Guard soldiers are barred from making formal arrests under the Posse Comitatus Act, though exceptions exist if the Insurrection Act is invoked, which has not yet occurred.