Homeland Security Secretary Michael Chertoff said authorities can try to manage risk of a mass transit attack and plan to respond effectively if something happens, but he added that they can’t eliminate the danger. “There is no perfect security in life, but we are constantly raising the baseline of our security,” Chertoff said on ABC’s “This Week.” Fran Townsend, President Bush’s homeland security adviser, said authorities have improved mass transit safety since Sept. 11, 2001, “but there are no guarantees in this world.” Townsend said the best tool against a similar explosion on a bus or train is to stop the bomb before it gets there. She said the counterterrorism legislation that passed after the Sept. 11 attacks helps authorities gather intelligence to prevent attacks, and she argued that the war in Iraq is keeping terrorists off U.S. public transportation. “If we’re waiting until the very last second, where an individual is going to strap on a bomb on or plant a bomb in a subway system, we’ve waited until a point where we’re least likely to be successful to prevent it,” she said in an appearance on “Fox News Sunday.” “That’s why you fight them away. That’s why you’re in Iraq and Afghanistan, fighting them there so you don’t have to fight them here.” While both Townsend and Chertoff said they are using intelligence to prevent an attack, both acknowledged that they had no warning that the London system was going to be hit Thursday. More than 49 people were killed in nearly simultaneous blasts in the subway system and another that tore apart a double-decker bus. Townsend expressed confidence that U.S. rail and bus passengers could help prevent such an attack.