LONG BEACH, Calif. (AP) — U.S. Homeland Security Secretary John Kelly says security at the nation’s ports is a top priority to preventing terrorism.

Kelly boarded a Coast Guard cutter in Long Beach on Thursday and watched a new unit conduct a demonstration responding to a mock radiological threat.

Members of the Coast Guard descended from helicopters with their rifles drawn and stormed the vessel as part of the exercise. Kelly watched from a deck above as they charged stairwells to search the ship.

Kelly says the threat to the nation’s ports is “always changing” and the U.S. will continue to train for evolving threats.