Clinton categorically denied any U.S. involvement in Mostafa Ahmadi Roshan's death.
Reports said two assailants on a motorcycle attached a magnetic bomb to Roshan's car, killing him and his driver Wednesday. Roshan was a chemistry expert and director of the Natanz uranium enrichment facility in central Iran.
Clinton called recent Iranian threats to close off the Persian Gulf "provocative and dangerous." She said the U.S. was committed to keeping the international waterway open. She called it "part of the lifeline that keeps oil and gas moving around the world."