San Francisco has joined a multi-city lawsuit against the U.S. Department of Homeland Security over its decision to halt a key counterterrorism grant program that supports detection and prevention of radiological and nuclear threats.

San Francisco joined Chicago, Boston, Denver, and Seattle in an amended complaint filed Tuesday. The lawsuit seeks to restore the congressionally approved funding and ensure DHS cannot unilaterally suspend critical homeland security support.

City Attorney David Chiu said in a statement that San Francisco is backing the legal challenge, originally filed by the city of Chicago, claiming that DHS unlawfully froze funding from the Securing the Cities program. The grant provides over $1 million annually to the San Francisco Bay Area for terrorism prevention efforts.

“The Securing the Cities grant allows jurisdictions across the country to prevent terrorist and nuclear attacks, yet the Trump Administration illegally yanked this funding with no explanation,” Chiu said in a statement. “Keeping our communities safe is our city’s top priority, and it should be the top priority of the Trump Administration as well.”

The STC program, created under a 2018 law signed by President Donald Trump during his first term, was designed to help urban areas develop the capability to detect and respond to nuclear and radiological threats. San Francisco’s portion of the grant supports 17 counties and cities across Northern California and parts of Nevada.

The city alleges DHS violated both the U.S. Constitution and the Administrative Procedure Act when it halted reimbursements and instructed jurisdictions to pause spending on detection equipment and supplies in May, citing “federal funding constraints.”

Local officials say the freeze threatens public safety, especially with major global events like Super Bowl LX and FIFA World Cup 2026 approaching. 

Mary Ellen Carroll, executive director of the city’s Department of Emergency Management, warned the funding is “essential to protecting the Bay Area from the radiological and nuclear events we all hope never occur.”

“When cities can no longer count on consistent administration of homeland security funding, our public safety suffers,” Carroll said.

The DHS did not immediately give a statement about the amended lawsuit.