(CNN) Far from ground zero in the Trump administration's crackdown on illegal immigration along the southern border, US Customs and Border Protection checkpoints on highways in Maine and New Hampshire are catching the eye of civil liberties groups.

On Interstate 95 near the remote northern Maine town of Lincoln this week, the Border Patrol said it made nine drug seizures and two arrests for immigration violations during an 11-hour checkpoint operation in which agents asked motorists about their place of birth and citizenship status.

The federal agency -- one of several at the center of a growing humanitarian crisis involving 2,300 children separated from their parents at the Mexico border -- said in a statement that Wednesday's checkpoint in Maine was "a means of preventing smuggling organizations from exploiting existing transportation systems to travel to the interior of the United States."

Customs and Border Protection says the US Supreme Court has affirmed the agency's ability to ask motorists' citizenship status, even if they have no suspicion. Agents use training and questions to make decisions about a traveler's citizenship or residency, it said.

"Travelers have the right to remain silent," it said in a statement. "Travelers who cooperate are passed through quickly, unless the agent suspects they are in violation of federal law. Travelers who refuse to cooperate may be referred to a secondary examination area to allow agents to conduct additional questioning to determine the traveler's citizenship or residency."