NEW: UK counter-terror police say a "significant operation" is underway as investigators pursue multiple lines of inquiry into the suspected terror plot surrounding a base used by the U.S. Air Force. Five British men have been arrested on suspicion of explosives and terrorism offenses as police examine vehicles found at the scene. No improvised explosives have been found. Authorities have also finished searching properties in London, but are not revealing further details as the investigation continues. The base has been used by the U.S. Air Force for decades, including in its campaign against Iran, as Secretary of State Marco Rubio says the situation clearly involved a foreign actor. @​StephBennettTV ▻