CHICAGO (CBS) — A security checkpoint Tuesday morning at the North Side Addison CTA station that worried some Red Line riders, who expressed their concerns on social media, was not related to immigration, according to a Chicago Police Department spokesman.

The Department of Homeland Security and the Chicago Police Public Transportation Section conducted the screening Tuesday morning, police spokesman Anthony Guglielmi said.

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The screenings are a counterterrorism effort that started in 2014, the Sun-Times reported at the time. Such checkpoints are conducted by officers who randomly search riders’ bags for explosives.

“At the request of the Chicago Transit Authority, TSA deployed its Visible Intermodal Prevention & Response (VIPR) team to provide additional prevention and response capabilities,” agency spokesman Michael McCarthy said in a statement.

“Security operations such as these are conducted throughout the nation in partnership with law enforcement and security authorities in all modes of transportation to include mass transit, rail, air and maritime.”

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They are conducted at subway stations between once a week and every few weeks. Guglielmi compared them to TSA screenings at the airports.

“They not only help to screen individuals, but give peace of mind for the traveling public to see the officers there, see the canines there,” Guglielmi said.

Some riders took to social media to express their concerns over the security checkpoint.

“CPD will never, in no way, ask for any type of immigration status,” Guglielmi said. “We have no involvement in that, we have no interest in that. The mayor has made it abundantly clear that Chicago is a sanctuary city. That has been communicated to every single officer.”

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Guglielmi said the checkpoints are also meant to encourage people to report suspicious activity and, if they see something, to say something.