WASHINGTON — A truck driven by terrorists goes down the streets of five large cities over two weeks quietly spraying anthrax spores, ultimately exposing 328,000 people and killing 13,300 while costing the economy billions of dollars.

It's a chilling possibility, one of 15 doomsday scenarios that Homeland Security authorities developed at the request of President Bush to better focus funding and to help state and local officials plan for terrorism and natural disasters.

The scenarios range from the most devastating — detonation of a 10-kiloton nuclear device in a big-city business district — to the unusual — the launch of the pneumonic plague in an airport bathroom, sports arena and train station.

While the scenarios are based on intelligence that terrorists have the capability and tradecraft to do such damage, an FBI official yesterday said there is no evidence that any such plots exist or that terrorists have the means to carry them out.

The nightmarish possibilities are found in a once-secret report that Homeland Security officials say came to light accidentally after it was posted on the Web by the state of Hawaii and reported by The New York Times yesterday.

Homeland Security Secretary Michael Chertoff said yesterday that it was a mistake for Hawaii to post the report, but it won't keep his agency from alerting state and local authorities about potential threats.

Chertoff said, however, that he plans to be less forthcoming with the public about possible terrorism threats as they unfold until he has definitive information to give.

The report was posted for more than three months on Hawaii's state Web site before officials took it down Tuesday night at Homeland Security's request.

"My understanding is that this was an error," Chertoff said.

Noting the report was in draft stages, Chertoff said Homeland Security wanted "a finished product out there. So that's unfortunate. But it's not going to deter us from working closely with our state and local partners in fashioning these plans."

Chertoff said Homeland Security will continue to share a wide array of information — including hunches, suspicions and tips — with local and state authorities. But he said he will "mightily resist the temptation" to give that same information to the public during potential attacks for fear of spreading inaccurate data.

Some experts and local officials said they are glad the report became public.

"I'm happy it's out in the open," said Stephen Flynn, national-security expert and director of a homeland-security task force at the Council of Foreign Relations. "We need to be much more open about these things and get engaged in a conversation about it."

Flynn said the United States has little good intelligence about al-Qaida, so it must instead look around society and decide what is most valuable and vulnerable. Hawaii officials noted the report was not labeled as confidential or classified. Moreover, the Hawaii officials said, Homeland Security never mentioned its sensitive nature in discussing the report during weekly conference calls.

"There was nothing on this document that was marked official use only," said Maj. Charles Anthony, a spokesman for the Hawaii Department of Defense. "There was nothing marked confidential."

Material from The Associated Press is included in this report.