U.S. Immigration and Customs Enforcement have been cracking down on undocumented immigrants, but on Monday the agency’s operations at Oakland Coliseum focused on unauthorized and unlicensed NFL gear, according to a Friday news release.

ICE targeted vendors of unauthorized T-shirts, hats, caps and bandannas. The agency said the raid was done in partnership with NFL brand security representatives and state and local law enforcement. The Oakland Police Department said it was not involved.

Officials said they seized about $11,000 worth of illegal swag — undoubtedly, most of it silver and black — during the “Monday Night Football” game and its pre- and postgame tailgate parties.

Tatum King, special agent in charge of the San Francisco Homeland Security Investigations unit, said about 400 pieces of merchandise were seized but no one was arrested. NFL brand officials issued warning letters and may be pursuing civil action, he said.

King said the investigations unit does not conduct immigration enforcement raids.

King said the agency is committed to ensuring the public purchases “legitimate products” instead of cheaper knockoffs often sold outside stadiums like the Coliseum.

“Oakland Raiders fans attending the first game of the season deserve genuine products and the proceeds of counterfeit merchandise ends up in the hands of transnational criminal organizations engaged in varying types of illegal activities,” King said.

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanatuan@sfchronicle.com Twitter: @ctuan