Ridge, in an address and question-and-answer session with publishers at The Associated Press annual meeting, said terrorism in America is a "permanent condition" and outlined goals for a long-term security strategy.
"We are at war; if we think there are only 20 terrorists -- the one we just caught and the 19 others" who attacked on Sept. 11, he said, "we are naive. We have got to believe there are more here."
For the first time, Ridge reviewed plans to release this summer or fall a national strategy to rank the nation's homeland defense needs. The plan will focus government resources where the risks are the highest, where most lives can be saved and most property can be protected, he said.
"It will reveal, in our judgment, what we need to protect," Ridge said. "It will outline the resources available to us and point the way for their best use."
Bioterrorism poses one of the greatest threats for massive loss of life, "and our preparedness has historically lagged behind the threat," Ridge said.
He told the news executives the homeland defense strategy "will answer two questions often asked by your reporters, and rightly so: 'Whose job is it -- and who pays for it?'"
Following the terrorist attacks, President Bush named the former Pennsylvania governor to be the White House point man for domestic-defense programs. The assignment touches on scores of federal activities, including border control, intelligence and safeguards against bioterrorism strikes.
Some in Congress want to give the position Cabinet-level status, which would grant lawmakers oversight power and, they say, increase Ridge's influence. Bush has balked, insisting that he has given Ridge enough power to overhaul homeland security from his working space just a few steps from the Oval Office.
Ridge recently unveiled a color-coded warning scheme to keep the nation on guard for terrorism. He told publishers that the warning system may be "tweaked" to adjust for suggestions raised during a public comment period.
Ridge was asked about the potential threat of terrorists getting their hands on nuclear weapons or exploding nuclear materials in a "dirty bomb." He replied that there is evidence the al-Qaida is seeking nuclear technology.
"If they obtain it one way or another, I don't have any doubt they will try to use it," he said.
In his address, Ridge said the fear of terrorism has receded for many Americans since Sept. 11.
"The world is just as dangerous today, if not more so," he said. "The threat is real; it's as real as it was seven months ago. In fact, it is a permanent condition to which this country must permanently adapt."
Ridge said his office is working with states and the private sector to study the nation's infrastructure and determine where the greatest risks are.
"The challenge is vast," he said. "It encompasses so much -- oil and gas refineries, power plants and electrical substations, water treatment plants and reservoirs, dams, pipelines, just to name a few. Add to that our schools and hospitals, our banks and financial institutions, our airports and seaports, our bridges and highways."
He said many communities are already at work securing potential targets and preparing emergency workers to respond to attacks.
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