Seattle is within 100 miles of border, meaning they have jurisdiction.
Via The Hill:
The Trump administration is sending a tactical border patrol team to Seattle, making good on President Trump’s pledge to use the full force of the federal government to protect federal property, The New York Times reports.
The team being sent to Seattle is similar to the teams that have already been deployed to Portland in an effort to fortify federal property that has been damaged by protestors that have dominated the city’s downtown area since the police killing of George Floyd in Minneapolis at the end of May.
Formally the federal officers are known as a Special Response Team and operate under U.S. Customs and Border Protection, according to the Times.